Water-absorbing agent and its production process and use

ABSTRACT

The present invention provides: a water-absorbing agent which has excellent urine resistance; a water-absorbing agent which has not only excellent urine resistance, but also excellent absorption properties that are stable to any composition of urine and show little change with time; and production processes and uses for these water-absorbing agents. The present invention water-absorbing agent exhibits a specific or larger value of absorption capacity under a load in a process in which the absorption capacity under a load is measured in a new manner using a specific liquid to be absorbed, and the present invention provides an absorbent matter and an absorbent article which display a specific or larger value of new absorption index as is, for example, led from the absorption capacity under a load or from the resin concentration using the above water-absorbing agent. The present invention further provides a production process for a water-absorbing agent having the above specific parameter.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to a water-absorbing agent and itsproduction process and use, more particularly, relates to awater-absorbing agent of excellent urine resistance, especially, awater-absorbing agent which can always exhibit excellent absorptionproperties regardless of the kinds of liquids, such as urine, to beabsorbed, and a production process for the water-absorbing agent, andfurther relates to uses of the water-absorbing agent, namely, toabsorbent matters and articles, and still further relates to anabsorption property measurement process by which absorption actions caneasily and precisely be predicted when the water-absorbing agent and theabsorbent matters and articles are practically used.

[0003] 2. Background Art

[0004] In recent years, water-absorbent resins (water-absorbing agents)are widely used as constituent materials of sanitary materials, such aspaper diapers, sanitary napkins, and so-called incontinent pads, for thepurpose of causing the water-absorbent resins to absorb body fluids suchas urine and menstrual blood.

[0005] Known examples of the above water-absorbent resins are asfollows: crosslinked polymers of partially neutralized polyacrylicacids; hydrolyzed products of starch-acrylic acid graft polymers;saponified products of vinyl acetate-acrylic acid ester copolymers;hydrolyzed products of acrylonitrile copolymers or acrylamidecopolymers, and their crosslinked polymers; and crosslinked polymers ofcationic monomers.

[0006] It is said that the above water-absorbent resins should, forexample, have the following properties: excellent water absorptionamount and speed, the gel strength, the suction power to suck up waterfrom a base material containing an aqueous liquid, upon contact withaqueous liquids such as body fluids. However, there are problems in thatrelations between these properties do not necessarily display positivecorrelations: for example, as the absorption capacity increases, someother properties such as liquid-permeability, gel strength, andabsorption speed deteriorate.

[0007] As to a method for improving such water-absorption properties ofthe water-absorbent resin in good balance, an art is known, in which theneighborhood of the surface of the water-absorbent resin is crosslinked,and various methods have been proposed as such.

[0008] For example, methods are known, in each of which the followingmaterials are used as the crosslinking agents: polyhydric alcohols(JP-A-58-180233 and JP-A-61-016903); polyglycidyl compounds,polyaziridine compounds, polyamine compounds, or polyisocyanatecompounds (JP-A-59-189103); polyvalent metals (JP-A-51-136588,JP-A-61-257235 and JP-A-62-007745); monoepoxy compounds(JP-A-61-098121); epoxy compounds and hydroxy compounds as used jointly(JP-A-02-132103); alkylene carbonates (DE 4020780).

[0009] However, there are problems in that: the balance between thewater-absorption properties is being improved by the above surfacetreatments, but when the water-absorbent resin is used for absorbentmatters of diapers, the water-absorbent resin deteriorates with time,and the liquid-permeability or the gel strength falls, so urine leaksfrom the diapers. The deterioration of the water-absorbent resin occursfrom the surface of the water-absorbent resin, and the soluble contentselute, and the liquid-permeability or the gel strength falls. Such adeterioration of the water-absorbent resin is considered to be caused bya very small amount of metal ion and by L-ascorbic acid as contained inurine.

[0010] By the way, the water-absorbent resin is powdery and thereforemight contain fine powders of 100 μm or less, and it is known to make agranulation by adding water for the purpose of improving the handlingability or the liquid-permeability in diapers. The granulation canprevent the powdering or improve the fluidity during moistureabsorption.

[0011] However, there are problems in that the granulation by addingwater to the surface-crosslinked water-absorbent resin facilitates thedestruction of the surface-crosslinked layer. Especially, as towater-absorbent resins with high absorption capacity under load asdesired in recent years, the elution of the soluble contents isprevented by crosslinking the neighborhood of the surface ofwater-absorbent resins with high absorption capacity, so the elution ofthe soluble contents cannot be suppressed in the case where thesurface-crosslinked layer is deteriorated by substances such asL-ascorbic acid when absorbing urine. Therefore, there are problems inthat when the water-absorbent resin is used for diapers, theliquid-permeability or the gel strength deteriorates, so urine leaksfrom the diapers.

[0012] On the other hand, as to uses of the water-absorbent resin, avariety of absorbent matters or articles using water-absorbent resinsare proposed, wherein the water-absorbent resins jointly have aplurality of the aforementioned properties and exhibit excellentperformance (water absorption properties) when used for sanitarymaterials such as paper diapers and sanitary napkins.

[0013] For example, the following are known: a water-absorbent resincomprising combinations of a gel capacity, a shear elastic modulus, andan extractive polymer content as are specified (U.S. Pat. No.4,654,039); a water-absorbent resin with a water absorption amount orspeed and a gel strength as are specified, and paper diapers andsanitary napkins using this water-absorbent resin (JP-A-60-185550,JP-A-60-185551, and JP-A-60-185804); paper diapers using awater-absorbent resin having a specific water absorption amount or speedand a gel stability (JP-A-60-185805); water-absorbent articles using awater-absorbent resin with a water absorption amount, a suction power,and a water-soluble content as are specified (JP-A-63-021902);water-absorbent sanitary supplies containing a water-absorbent resinwith a water absorption amount, a water absorption amount under a load,and a gel fracture strength as are specified (JP-A-63-099861); paperdiapers containing a water-absorbent resin with a water absorptionamount and a water absorption speed under a load as are specified(JP-A-02-034167); a water-absorbing agent containing a water-absorbentresin with a water absorption amount under a load and a particlediameter as are specified (EP 339,461); a water-absorbing agentcontaining a specific or larger amount of water-absorbent resin with awater absorption speed and a water absorption amount under a load in ashort time as are specified (EP 443,627); a water-absorbent combinedmaterial containing a specific or larger amount of water-absorbent resinwith a deformation under a load and a suction index as are specified (EP532,002); and an absorbent article using a resin with a pressureabsorption index and a 16-hour extractability level as are regulated (EP615,735).

[0014] In recent years, absorbent articles such as paper diapers aregetting thinner and thinner, and the amount of the water-absorbentresin, as used for an absorbent layer of the absorbent articles, tendsto increase. That is to say, as to the above absorbent layer, what hasthe weight ratio of 0.3 or more, particularly, 0.5 or more, of thewater-absorbent resin to the total of the water-absorbent resin and thefibrous base material (this ratio might hereinafter be referred to as“resin concentration”) is becoming the mainly current. However, it isbecoming clear that there are, still, problems when the above alreadyknown resins with a variety of regulated properties are used for theseabsorbent articles having high resin concentration. That is to say, thewater absorption properties of the absorbent articles are being improvedby combinations of the above various properties, but it is being closedup that there are problems in that, depending on the composition of theliquid to be absorbed, the water absorption properties of the resinscannot be sufficiently displayed especially when the resin concentrationin the absorbent articles is high. It is being said that there areproblems in that, when the absorbent article is, for example, a paperdiaper, the composition of urine varies with factors, such as users'ages, taken food and drink, prescribed medicines, and in that theabsorption action of the water-absorbent resin might therefore begreatly different from expectation.

SUMMARY OF THE INVENTION

[0015] A. Objects of the Invention

[0016] Therefore, an object of the present invention is to provide: awater-absorbing agent which undergoes little deterioration with timewhen absorbing urine and thus has excellent urine resistance; awater-absorbing agent which has not only excellent urine resistance, butalso absorption properties that are stable to any composition of urineand show little change with time, and which is therefore used especiallyfavorably for absorbent articles having high resin concentration; andproduction processes for these water-absorbing agents.

[0017] In addition, another object of the present invention is toclarify what absorption properties are needed for water-absorbent resinswhen the resin ratio is a specific value, and to provide an absorbentarticle using the optimum water-absorbent resin for each water-absorbentresin ratio, and to provide an absorbent matter and an absorbentarticle, both of which display an always stable high absorption amount,especially, a high absorption amount till the leakage occurs in a usedstate very near to practical use.

[0018] In addition, yet another object of the present invention is toprovide an absorption property measurement process by which absorptionactions can easily and precisely be predicted when the water-absorbingagent and the absorbent matters and articles are practically used, andwhich is very useful for producing a water-absorbing agent, absorbentmatter, or absorbent article that displays excellent absorptionproperties.

[0019] B. Disclosure of the Invention

[0020] The present inventors studied and studied with encouragement tothemselves and with great efforts to achieve the above object. As aresult, the present inventors completed the present invention bydeveloping new evaluation processes for (1) a deterioration absorptioncapacity under a load as seen using a specific liquid to be absorbed,(2) a deterioration absorption capacity under a load as seen afterexecution of specific procedure using a specific liquid to be absorbed,and (3) a deterioration absorption index under a load, and by findingthat the above problems could be solved by a water-absorbing agent whichexhibits a specific or larger value of absorption capacity ordeterioration absorption index under a load in these evaluationprocesses. Parameter (1) above is not provided with the specificprocedure, so it is hereinafter referred to as static deteriorationabsorption capacity under a load and includes four stages (1), (2), (3),and (4) in view of the largeness of the load, and particularly, stages(1) and (4) are important. Parameter (2) above is provided with thespecific procedure, so it is hereinafter referred to as dynamicdeterioration absorption capacity under a load.

[0021] Then, the present inventors found a process for obtaining awater-absorbing agent that displays the above specific absorptioncapacities or index (hereinafter, these might generically be referred toas parameters), in which an ion blocking or chelating agent including anamino polycarboxylic acid is preferably added to a water-absorbent resinby a specific method.

[0022] In addition, the present inventors studied and studied withencouragement to themselves and with great efforts about relationsbetween the resin ratio in the absorbent matter and the physicalproperties of the absorbing agent. As a result, the present inventorscompleted the present invention by finding that the absorption amount,standing till the occurrence of the leakage in a used state very near topractical use, depends on specific relations as led from properties ofthe water-absorbing agent, such as absorption capacity under no load andthe above new specific absorption capacities or index under a load, andfrom the resin ratio in the absorbent matter, and that the absorptionamount of the absorbent matter or article in a used state very near topractical use increases if the water-absorbing agent and the resin ratioare selected so as to enlarge values of formulae of the above relations.

[0023] The water-absorbing agent, according to the present invention,can be any one of 1˜3 below.

[0024] 1. A water-absorbing agent, having an absorption capacity of 30(g/g) or more under no load and static deterioration absorption capacity(1) of 20 (g/g) or more under a load.

[0025] 2. A water-absorbing agent, having an absorption capacity of 30(g/g) or more under no load and a dynamic deterioration absorptioncapacity of 20 (g/g) or more under a load.

[0026] 3. A water-absorbing agent, having an absorption capacity of 30(g/g) or more under no load and static deterioration absorption capacity(4) of 23 (g/g) or more under a load.

[0027] An absorbent matter, according to the present invention,comprises the above present invention water-absorbing agent and afibrous base material, wherein the weight ratio of the water-absorbingagent to the total of the water-absorbing agent and the fibrous basematerial is 0.4 or more.

[0028] An absorbent article, according to the present invention,comprises:

[0029] an absorbent layer including the above present inventionabsorbent matter;

[0030] a liquid-permeable surface sheet; and

[0031] a liquid-impermeable back sheet.

[0032] An absorption property measurement process, according to thepresent invention, is characterized in that a liquid containing areducible substance is used as a liquid to be absorbed in a process formeasuring at least one absorption property selected from the groupconsisting of: absorption properties under a load of a water-absorbingagent; absorption properties of an absorbent matter of which the weightratio of a water-absorbing agent to the total of the water-absorbingagent and a fibrous base material is 0.4 or more; and absorptionproperties of an absorbent article including the above absorbent matter.

[0033] A production process for a water-absorbing agent, according tothe present invention, comprises the step of mixing an ion blockingagent and a surface-crosslinking agent, which is reactable upon acarboxyl group, with a water-absorbent resin having a carboxyl group.

[0034] Another production process for a water-absorbing agent, accordingto the present invention, comprises the steps of:

[0035] crosslinking the neighborhood of the surface of a water-absorbentresin which is obtained by polymerizing a monomer component including anunsaturated carboxylic acid in the presence of an internal-crosslinkingagent; and

[0036] adding water and an ion blocking agent to the resultantsurface-crosslinked water-absorbent resin, thus granulating thewater-absorbent resin.

[0037] Yet another water-absorbing agent, according to the presentinvention, is obtained by a process including the step of adding to awater-absorbent resin at least one chelating agent selected from thegroup consisting of compounds of general formulae (1) and (2) and maleichydrophilic polymers (including salts) (3),

[0038] wherein general formula (1) is:

[0039] wherein: n, X¹, and R¹˜R³ denote the following numbers andstructures:

[0040] and wherein general formula (2) is:

[0041] wherein: m, X², and R⁵˜R⁸ denote the following numbers andstructures:

[0042] The above and other objects and the advantages of the presentinvention will be more fully apparent from the following detaileddisclosure.

BRIEF DESCRIPTION OF THE DRAWING

[0043]FIG. 1 illustrates a measurement apparatus for the waterabsorption capacity under a load.

DETAILED DESCRIPTION OF THE INVENTION

[0044] Hereinafter, the present invention is explained in detail.

[0045] Water-Absorbing Agent

[0046] The water-absorbing agent of the present invention has a specificor larger value of absorption capacity under no load and further hasspecific or larger values with respect to the following new properties:static deterioration absorption capacity under a load, dynamicdeterioration absorption capacity under a load, and deteriorationabsorption index under a load.

[0047] The absorption capacity under no load in the present invention isa numerical value as calculated by a method in which: 0.2 g ofwater-absorbing agent is uniformly placed into a nonwoven-fabric-madebag (60 mm×60 mm) and then immersed into a 0.9 wt % aqueous sodiumchloride solution (physiological sodium chloride solution); sixtyminutes later, the bag is drawn up and then drained at 250 G for 3minutes with a centrifuge, and the weight W₁ (g) of the bag is thenmeasured; on the other hand, the same procedure is carried out using nowater-absorbing agent, and the resultant weight W₀ (g) is measured;thus, the absorption capacity is calculated from the above weights W₁and W₀ and the weight of the water-absorbing agent in accordance withthe following equation:

absorption capacity (g/g)={(weight W ₁−weight W ₀)/(weight ofwater-absorbing agent)}−1.

[0048] The static deterioration absorption capacity under a load in thepresent invention is an absorption capacity that is measured under aload for a water-absorbing agent (resin) after carrying out a treatmentin which: the water-absorbing agent is swollen to 15 times with aphysiological sodium chloride solution containing L-ascorbic acid in apredetermined concentration as the liquid to be absorbed, and then theswollen agent is allowed to stand stationary for a predetermined time.This static deterioration absorption capacity under a load is a newevaluation item for a water-absorbing agent.

[0049] The following absorption properties of conventionalwater-absorbent resins (water-absorbing agents) are, for example, known:absorption capacity, absorption capacity under a load,liquid-permeability, suction power, and absorption speed. However, themeasurement is generally made in a comparatively short period of timeusing a liquid with an electrolyte concentration near that of urine.However, in many cases, the actual wearing time of diapers extends for along time of 6 hours or more. Therefore, water-absorbent resins, whichprovide excellent results with regard to the above conventionalevaluation items as have been proposed so far, do not necessarilyexhibit excellent performance in practical use as well. In addition,urine contains compounds which change (deteriorate) the properties ofthe resin with time, and the existence of these compounds also largelyinfluences the absorption actions of the water-absorbent resin inpractical use.

[0050] The present inventors studied and studied with encouragement tothemselves and with great efforts to develop an evaluation process whichcan rightly evaluate the absorption abilities of the water-absorbentresin in practical use. As a result, the present inventors found thatthe absorption actions in practical use can easily and precisely bepredicted by measuring the absorption capacity under a load afterallowing the water-absorbent resin to stand stationary for acomparatively long period of time in a physiological sodium chloridesolution containing L-ascorbic acid in a predetermined concentration asthe liquid to be absorbed.

[0051] Conventional processes are known, in which the water-absorbentresin is solubilized using L-ascorbic acid or its salts, or the amountof soluble component as solubilized in such a way is measured (e.g.JP-A-05-247221, JP-A-07-059813, JP-A-08-337726, JP-A-10-067805).However, in these techniques, the water-absorbent resin is solubilizedunder saturation-swelling conditions, and nothing is considered inrespect to how the ability to absorb a liquid, which is the inherentrole of the water-absorbent resin, changes when the resin is used,whereas the static deterioration absorption capacity under a load in thepresent invention is a new evaluation item which enables judgment of howthe inherent absorption abilities remaining in the resin that onceabsorbed urine will change due to urine until the resin further absorbsurine next time.

[0052] Static deterioration absorption capacity (1) under a load in thepresent invention is an absorption capacity of the water-absorbing agentas determined by the following sequential steps of:

[0053] forming a water-absorbing agent as swollen to 15 (g/g) with aphysiological sodium chloride solution containing L-ascorbic acid in aconcentration of 0.005 weight %;

[0054] leaving the water-absorbing agent in such a swollen state for 6hours;

[0055] allowing the swollen water-absorbing agent to absorb thephysiological sodium chloride solution for another 1 hour in a statewhere a load of 50 g/cm² is mounted on the swollen water-absorbingagent; and

[0056] measuring the weight of the resultant swollen gel.

[0057] The water-absorbing agent of the present invention ischaracterized by having an absorption capacity of 30 (g/g) or more underno load and the above static deterioration absorption capacity (1) of 20(g/g) or more under a load. In the case where the absorption capacityunder no load is less than 30 (g/g), the absorption abilities areinsufficient, and the leakage and so on are apt to occur especially whenthe water-absorbing agent is used for absorbent articles having highresin concentration. The absorption capacity under no load is preferablyat least 33 (g/g), more preferably at least 35 (g/g). In addition, inthe case where static deterioration absorption capacity (1) under a loadis less than 20 (g/g), similarly, the absorption abilities of theabsorbent article are insufficient, and the leakage and so on are apt tooccur, or the absorption actions greatly vary due to factors such ascomposition changes of liquids to be absorbed, so the stable absorptionproperties are not obtainable. Static deterioration absorption capacity(1) under a load is preferably 23 (g/g) or more.

[0058] Static deterioration absorption capacity (2) under a load in thepresent invention is an absorption capacity of the water-absorbing agentas determined by the following sequential steps of:

[0059] forming a water-absorbing agent as swollen to 15 (g/g) with aphysiological sodium chloride solution containing L-ascorbic acid in aconcentration of 0.005 weight %;

[0060] leaving the water-absorbing agent in such a swollen state for 2hours;

[0061] allowing the swollen water-absorbing agent to absorb thephysiological sodium chloride solution for another 1 hour in a statewhere a load of 50 g/cm² is mounted on the swollen water-absorbingagent; and

[0062] measuring the weight of the resultant swollen gel.

[0063] The water-absorbing agent of the present invention ischaracterized by having an absorption capacity of 30 (g/g) or more underno load and the above static deterioration absorption capacity (2) of 23(g/g) or more under a load. In the case where the absorption capacityunder no load is less than 30 (g/g), the absorption abilities areinsufficient, and the leakage and so on are apt to occur especially whenthe water-absorbing agent is used for absorbent articles having highresin concentration. The absorption capacity under no load is preferablyat least 33 (g/g), more preferably at least 35 (g/g). In addition, inthe case where static deterioration absorption capacity (2) under a loadis less than 23 (g/g), similarly, the absorption abilities of theabsorbent article are insufficient, and the leakage and so on are apt tooccur, or the absorption actions greatly vary due to factors such ascomposition changes of liquids to be absorbed, so the stable absorptionproperties are not obtainable. Static deterioration absorption capacity(2) under a load is preferably 25 (g/g) or more.

[0064] Static deterioration absorption capacity (3) under a load in thepresent invention is an absorption capacity of the water-absorbing agentas determined by the following sequential steps of:

[0065] forming a water-absorbing agent as swollen to 15 (g/g) with aphysiological sodium chloride solution containing L-ascorbic acid in aconcentration of 0.05 weight %;

[0066] leaving the water-absorbing agent in such a swollen state for 2hours;

[0067] allowing the swollen water-absorbing agent to absorb thephysiological sodium chloride solution for another 1 hour in a statewhere a load of 50 g/cm² is mounted on the swollen water-absorbingagent; and

[0068] measuring the weight of the resultant swollen gel.

[0069] The water-absorbing agent of the present invention ischaracterized by having an absorption capacity of 30 (g/g) or more underno load and the above static deterioration absorption capacity (3) of 20(g/g) or more under a load. In the case where the absorption capacityunder no load is less than 30 (g/g), the absorption abilities areinsufficient, and the leakage and so on are apt to occur especially whenthe water-absorbing agent is used for absorbent articles having highresin concentration. The absorption capacity under no load is preferablyat least 33 (g/g), more preferably at least 35 (g/g). In addition, inthe case where static deterioration absorption capacity (3) under a loadis less than 20 (g/g), similarly, the absorption abilities of theabsorbent article are insufficient, and the leakage and so on are apt tooccur, or the absorption actions greatly vary due to factors such ascomposition changes of liquids to be absorbed, so the stable absorptionproperties are not obtainable. Static deterioration absorption capacity(3) under a load is preferably 23 (g/g) or more.

[0070] Static deterioration absorption capacity (4) under a load in thepresent invention is an absorption capacity of the water-absorbing agentas determined by the following sequential steps of:

[0071] forming a water-absorbing agent as swollen to 15 (g/g) with aphysiological sodium chloride solution containing L-ascorbic acid in aconcentration of 0.05 weight %;

[0072] leaving the water-absorbing agent in such a swollen state for 6hours;

[0073] allowing the swollen water-absorbing agent to absorb thephysiological sodium chloride solution for another 1 hour in a statewhere a load of 20 g/cm² is mounted on the swollen water-absorbingagent; and

[0074] measuring the weight of the resultant swollen gel.

[0075] The water-absorbing agent of the present invention ischaracterized by having an absorption capacity of 30 (g/g) or more underno load and the above static deterioration absorption capacity (4) of 30(g/g) or more under a load. In the case where the absorption capacityunder no load is less than 30 (g/g), the absorption abilities areinsufficient, and the leakage and so on are apt to occur especially whenthe water-absorbing agent is used for absorbent articles having highresin concentration. The absorption capacity under no load is preferablyat least 33 (g/g), more preferably at least 35 (g/g). In addition, inthe case where static deterioration absorption capacity (4) under a loadis less than 30 (g/g), similarly, the absorption abilities of theabsorbent article are insufficient, and the leakage and so on are apt tooccur, or the absorption actions greatly vary due to factors such ascomposition changes of liquids to be absorbed, so the stable absorptionproperties are not obtainable. Static deterioration absorption capacity(4) under a load is preferably at least 32 (g/g), more preferably atleast 34 (g/g).

[0076] The present invention provides a new water-absorbing agent ofwhich the above absorption capacity under no load and staticdeterioration absorption capacities (1), (2), (3), (4) under a load arespecific or larger values. Such a water-absorbing agent is favorablyused even for paper diapers which have high resin concentration and lowfibrous-base-material concentration accompanying the thinning of paperdiapers in recent years, and this agent further can reduce the leakagein practical use.

[0077] The present inventors found that the measurement value of staticdeterioration absorption capacity (1) or (4) under a load was especiallyimportant. Thus the present invention provides a new water-absorbingagent of which the absorption capacity under no load and staticdeterioration absorption capacity (1) or (4) under a load are specificor larger values, and the absorbent article (e.g. paper diaper) usingthe present invention water-absorbing agent can reduce the leakage inpractical use.

[0078] The measurement value of static deterioration absorption capacity(1) under a load of the water-absorbing agent is important for paperdiapers which have high resin concentration and lowfibrous-base-material concentration accompanying the thinning of paperdiapers in recent years.

[0079] The dynamic deterioration absorption capacity under a load in thepresent invention is an absorption capacity that is measured under aload for a water-absorbing agent (resin) after carrying out a treatmentin which: the water-absorbing agent is swollen to 15 times with aphysiological sodium chloride solution containing L-ascorbic acid in apredetermined concentration as the liquid to be absorbed, and then theswollen agent is allowed to stand stationary for a predetermined timeand then dynamically damaged supposing movements in practical use. Thisstatic deterioration absorption capacity under a load is a newevaluation item for a water-absorbing agent.

[0080] The following absorption properties of conventionalwater-absorbent resins (water-absorbing agents) are, for example, known:absorption capacity, absorption capacity under a load,liquid-permeability, suction power, and absorption speed. In addition, amethod is known, in which: the water-absorbent resin is allowed toabsorb a physiological sodium chloride solution and to thereby gelate,and the resultant gel is sheared, and then the re-absorption ability ofthe gel is measured (U.S. Pat. No. 5,453,323). However, the measurementof the above properties is generally made in a comparatively shortperiod of time using a liquid with an electrolyte concentration nearthat of urine. Therefore, water-absorbent resins that provide excellentevaluation results do not necessarily exhibit excellent performance inpractical use as well. In addition, urine contains compounds whichchange (deteriorate) the properties of the resin with time, and theexistence of these compounds also largely influences the absorptionactions of the water-absorbent resin in practical use. Furthermore,because users move in practical use, dynamic force as well as load actsupon the resin.

[0081] The present inventors studied and studied with encouragement tothemselves and with great efforts to develop an evaluation process whichcan rightly evaluate the absorption abilities of the water-absorbentresin in practical use. As a result, the present inventors found thatthe absorption actions in practical use can easily and precisely bepredicted by measuring the absorption capacity under a load aftercarrying out a treatment in which: the water-absorbent resin is allowedto stand stationary for a comparatively long period of time in aphysiological sodium chloride solution containing L-ascorbic acid in apredetermined concentration as the liquid to be absorbed, and then theresin is subjected to dynamic force.

[0082] Conventional processes are known, in which the water-absorbentresin is solubilized using L-ascorbic acid or its salts, or the amountof soluble component as solubilized in such a way is measured (e.g.JP-A-05-247221, JP-A-07-059813, JP-A-08-337726, JP-A-10-067805).However, in these techniques, the water-absorbent resin is solubilizedunder saturation-swelling conditions, and nothing is considered inrespect to how the ability to absorb a liquid, which is the inherentrole of the water-absorbent resin, changes when the resin is used,whereas the dynamic deterioration absorption capacity under a load inthe present invention is a new evaluation item which enables judgment ofhow the inherent absorption abilities remaining in the resin that onceabsorbed urine will change due to urine and dynamic force, as applied tothe resin, until the resin further absorbs urine next time.

[0083] The dynamic deterioration absorption capacity under a load in thepresent invention is an absorption capacity of the water-absorbing agentas determined by the following sequential steps of:

[0084] forming a water-absorbing agent as swollen to 15 (g/g) with aphysiological sodium chloride solution containing L-ascorbic acid in aconcentration of 0.005 weight %;

[0085] leaving the water-absorbing agent in such a swollen state for 4hours;

[0086] dynamically damaging the swollen water-absorbing agent;

[0087] allowing the dynamically damaged water-absorbing agent to absorbthe physiological sodium chloride solution for another 1 hour in a statewhere a load of 50 g/cm² is mounted on the swollen water-absorbingagent; and

[0088] measuring the weight of the resultant swollen gel.

[0089] The water-absorbing agent of the present invention ischaracterized by having an absorption capacity of 30 (g/g) or more underno load and the above dynamic deterioration absorption capacity of 20(g/g) or more under a load. In the case where the absorption capacityunder no load is less than 30 (g/g), the absorption abilities areinsufficient, and the leakage and so on are apt to occur especially whenthe water-absorbing agent is used for absorbent articles with high resinconcentration. The absorption capacity under no load is preferably atleast 33 (g/g), more preferably at least 35 (g/g). In addition, in thecase where the dynamic deterioration absorption capacity under a load isless than 20 (g/g), similarly, the absorption abilities of the absorbentarticle are insufficient, and the leakage and so on are apt to occur, orthe absorption actions greatly vary due to factors, such as compositionchanges of liquids to be absorbed and dynamic force as applied to theresin, so the stable absorption properties are not obtainable. Thedynamic deterioration absorption capacity under a load is preferably 23(g/g) or more.

[0090] The present invention provides a new water-absorbing agent ofwhich the above absorption capacity under no load and the dynamicdeterioration absorption capacity under a load are specific or largervalues. Such a water-absorbing agent is favorably used even for paperdiapers which have high resin concentration and lowfibrous-base-material concentration accompanying the thinning of paperdiapers in recent years, and this agent further can reduce the leakagein practical use.

[0091] The deterioration absorption index under a load in the presentinvention is the total of the above static deterioration absorptioncapacities (1)˜(4) and dynamic deterioration absorption capacity under aload. The deterioration absorption index under a load is an item ofevaluation in which damage in practical use is supposed. It isconsidered that as the total of the values as obtained by the aboveitems of evaluation gets larger, the water-absorbing agent alwaysdisplays higher performance even if subjected to a variety of damage asproduced in practical use.

[0092] The water-absorbing agent of the present invention has anabsorption capacity of 30 (g/g) or more under no load and the abovedeterioration absorption index of 110 (g/g) or more under a load. In thecase where the absorption capacity under no load is less than 30 (g/g),the absorption abilities are insufficient, and the leakage and so on areapt to occur especially when the water-absorbing agent is used forabsorbent articles with high resin concentration. The absorptioncapacity under no load is preferably at least 33 (g/g), more preferablyat least 35 (g/g). In addition, in the case where the deteriorationabsorption index under a load is less than 110 (g/g), similarly, theabsorption abilities of the absorbent article are insufficient, and theleakage and so on are apt to occur, or the absorption actions greatlyvary due to factors, such as composition changes of liquids to beabsorbed and dynamic force as applied to the resin, so the stableabsorption properties are not obtainable. The deterioration absorptionindex under a load is preferably at least 120 (g/g), more preferably atleast 130 (g/g).

[0093] The present invention provides a new water-absorbing agent ofwhich the above absorption capacity under no load and the deteriorationabsorption index under a load are specific or larger values. Such awater-absorbing agent is favorably used even for paper diapers whichhave high resin concentration and low fibrous-base-materialconcentration accompanying the thinning of paper diapers in recentyears, and this agent further can reduce the leakage in practical use.

[0094] As is aforementioned, the present inventors studied and studiedwith encouragement to themselves and with great efforts to develop anevaluation process which can rightly evaluate the absorption abilitiesof the water-absorbent resin in practical use, so that the inventorsfound new properties of the above static deterioration absorptioncapacities under a load, dynamic deterioration absorption capacity undera load, and deterioration absorption index under a load, but theinventors further found that the absorption actions in practical use caneasily be predicted to some extent by swelling the resin with aphysiological sodium chloride solution, and then allowing the resin tostand stationary for a long time, and then measuring the absorptioncapacity under a load (i.e. substantial absorption capacity under aload).

[0095] That is to say, it is possible to evaluate the absorptionabilities of the water-absorbent resin as displayed in the case wherethe amount of components which deteriorate the water-absorbent resin issmall in practical use or where variation of urine does not very greatlyoccur in practical use. However, if it is considered that, in practicaluse, the amount of components which deteriorate the water-absorbentresin is liable to be large or variation of urine tends to occur, thenthe new property of the above substantial absorption capacity under aload seems to be an absorption ability of the water-absorbent resin asis at least needed in practical use.

[0096] This substantial absorption capacity under a load is anabsorption capacity that is measured under a load for a water-absorbingagent (resin) after carrying out a treatment in which: thewater-absorbing agent is swollen to 15 times with a physiological sodiumchloride solution as the liquid to be absorbed, and then the swollenagent is allowed to stand stationary for a predetermined time. Thissubstantial absorption capacity under a load is a new evaluation itemfor a water-absorbing agent. The below-mentioned two substantialabsorption capacities (1) and (2) under a load are exemplified inaccordance with the duration for which the swollen water-absorbing agentis allowed to stand stationary. These absorption capacities (1) and (2)under a load enable judgment of how the inherent absorption abilitiesremaining in the resin that once absorbed urine will change due to urineuntil the resin further absorbs urine next time.

[0097] To begin with, substantial absorption capacity (1) under a loadin the present invention is an absorption capacity of thewater-absorbing agent as determined by the following sequential stepsof:

[0098] forming a water-absorbing agent as swollen to 15 (g/g) with aphysiological sodium chloride solution;

[0099] leaving the water-absorbing agent in such a swollen state for 2hours;

[0100] allowing the swollen water-absorbing agent to absorb thephysiological sodium chloride solution for another 1 hour in a statewhere a load of 50 g/cm² is mounted on the swollen water-absorbingagent; and

[0101] measuring the weight of the resultant swollen gel.

[0102] In the present invention, it is preferable that thewater-absorbing agent has an absorption capacity of 30 (g/g) or moreunder no load and the above substantial absorption capacity (1) of 23(g/g) or more under a load. In this case, it is permissible that thewater-absorbing agent has any one or two or more of the new propertiesof the above specific or larger values of static deteriorationabsorption capacities (1)˜(4) under a load, dynamic deteriorationabsorption capacity under a load, and deterioration absorption indexunder a load. In the case where the absorption capacity under no load isless than 30 (g/g), the absorption abilities are insufficient, and theleakage and so on are apt to occur especially when the water-absorbingagent is used for absorbent articles having high resin concentration.The absorption capacity under no load is preferably at least 33 (g/g),more preferably at least 35 (g/g). In addition, in the case wheresubstantial absorption capacity (1) under a load is less than 23 (g/g),similarly, the absorption abilities of the absorbent article areinsufficient, and the leakage and so on are apt to occur, and the stableabsorption properties are not obtainable. Substantial absorptioncapacity (1) under a load is preferably at least 24 (g/g), morepreferably at least 25 (g/g).

[0103] Next, substantial absorption capacity (2) under a load in thepresent invention is an absorption capacity of the water-absorbing agentas determined by the following sequential steps of:

[0104] forming a water-absorbing agent as swollen to 15 (g/g) with aphysiological sodium chloride solution;

[0105] leaving the water-absorbing agent in such a swollen state for 6hours;

[0106] allowing the swollen water-absorbing agent to absorb thephysiological sodium chloride solution for another 1 hour in a statewhere a load of 50 g/cm² is mounted on the swollen water-absorbingagent; and

[0107] measuring the weight of the resultant swollen gel.

[0108] In the present invention, it is also preferable that thewater-absorbing agent has an absorption capacity of 30 (g/g) or moreunder no load and the above substantial absorption capacity (2) of 20(g/g) or more under a load. Also in this case, it is permissible thatthe water-absorbing agent has any one or two or more of the newproperties of the above specific or larger values of staticdeterioration absorption capacities (1)˜(4) under a load, dynamicdeterioration absorption capacity under a load, and deteriorationabsorption index under a load. In the case where the absorption capacityunder no load is less than 30 (g/g), the absorption abilities areinsufficient, and the leakage and so on are apt to occur especially whenthe water-absorbing agent is used for absorbent articles having highresin concentration. The absorption capacity under no load is preferablyat least 33 (g/g), more preferably at least 35 (g/g). In addition, inthe case where substantial absorption capacity (2) under a load is lessthan 20 (g/g), similarly, the absorption abilities of the absorbentarticle are insufficient, and the leakage and so on are apt to occur,and the stable absorption properties are not obtainable. Substantialabsorption capacity (2) under a load is preferably at least 23 (g/g).

[0109] The present invention can further provide a new water-absorbingagent which preferably has specific or larger values of absorptioncapacity under no load and substantial absorption capacities (1)˜(2)under a load. Such a water-absorbing agent is favorably used even forpaper diapers which have high resin concentration and lowfibrous-base-material concentration accompanying the thinning of paperdiapers in recent years, and this agent further can reduce the leakagein practical use.

[0110] The water-absorbing agent of the present invention preferably hasan absorption speed of 20˜80 (sec) and a water-soluble content of 1˜15weight %. The water-soluble content is in the range of preferably 2˜15weight %, more preferably 2˜10 weight %. In the case where theabsorption speed exceeds 80 (sec), the absorption of liquid by absorbentmatters or articles including the water-absorbing agent is so slow until60 minutes pass that a large amount of liquid tends to be desorbed. Inthe case where the absorption speed is less than 20 (sec), theabsorption of liquid by absorbent matters or articles including thewater-absorbing agent is so excessively fast that gel blocks easilyform. These phenomena greatly occur especially to absorbent matters orarticles with high weight ratio (resin concentration) of thewater-absorbing agent to the total of the water-absorbing agent and thefibrous base material. In addition, a water-absorbing agent with awater-soluble content less than 1 weight % costs very high for itsproduction and is therefore difficult to produce, and further, when thewater-soluble content is reduced, the absorption capacity under no loadusually tends to fall. In the case where the water-soluble content ismore than 15 weight %, it is difficult to obtain the water-absorbingagent with the static deterioration absorption capacities under a load,dynamic deterioration absorption capacity under a load, anddeterioration absorption index under a load falling in the scope of thepresent invention, or it is also difficult to obtain the water-absorbingagent with the substantial absorption capacities under a load falling inthe aforementioned preferable range.

[0111] As to the composition of the present invention water-absorbingagent, what includes a water-absorbent resin as an essential componentis preferably used.

[0112] The present invention water-absorbing agent with theaforementioned specific parameters is, for example, obtainable by eitherone of the following processes:

[0113] 1. a process in which a specific amino polycarboxylic acid and asurface-crosslinking agent that is reactive upon the carboxyl group of awater-absorbent resin are mixed with the water-absorbent resin tocrosslink this resin;

[0114] 2. a process in which a specific amino polycarboxylic acid isadded to a specific surface-crosslinked water-absorbent resin having anabsorption capacity of 23 (g/g) or more under a load.

[0115] However, the process for obtaining the present inventionwater-absorbing agent is not limited to the above-mentioned ones.

[0116] Hereinafter, the production process for the water-absorbingagent, according to the present invention, is explained in detail.

[0117] The water-absorbent resin, which is used to produce thewater-absorbing agent of the present invention, is a conventionallyknown resin that absorbs as large a quantity of water as 50˜1,000 timesthe original in ion-exchange water to thereby form a hydrogel. Examplesof such a water-absorbent resin include: crosslinked polymers ofpartially neutralized polyacrylic acids; hydrolyzed products ofstarch-acrylonitrile graft polymers; hydrolyzed products ofstarch-acrylic acid graft polymers; saponification products of vinylacetate-acrylic acid ester copolymers; hydrolyzed products ofacrylonitrile copolymers or acrylamide copolymers, or their crosslinkedpolymers; saponified products of crosslinked polyvinyl alcoholscontaining a carboxylic group; and crosslinked isobutylene-maleicanhydride copolymers. Among them, those which have a carboxylic groupare preferable and typically obtained by polymerizing and crosslinkingmonomers of which the main component is acrylic acid and/or a salt(neutralized product) thereof. In addition, as to the abovewater-absorbent resin, those which have an uncrosslinked water-solublecontent of 25 weight % or below, preferably 15 weight % or below, morepreferably 10 weight % or below, are used. The carboxyl group content inthe water-absorbent resin is not especially limited, but is preferably0.01 equivalents or more per 100 g of the water-absorbent resin. Forexample, the neutralization ratio of the polyacrylic acid is in therange of desirably 1˜60 mol %, more desirably 10˜50 mol %.

[0118] Examples of the above salt of acrylic acid include: alkalinemetal salts (e.g. salts of sodium, potassium, and lithium), ammoniumsalts, and amine salts of acrylic acid. It is preferable that theconstituent units of the above water-absorbent resin comprise acrylicacid of 0˜50 mol %, more preferably 10˜40 mol %, and its salt of 100˜50mol %, more preferably 90˜60 mol %, (wherein the total of both is 100mol %). The neutralization may be carried out either to monomers beforepolymerization or to the resultant polymer during or afterpolymerization, but is preferably carried out to monomers beforepolymerization in view of production cost, because neutralization of thepolymer needs a considerably long time.

[0119] The monomers to produce the water-absorbent resin of the presentinvention may further comprise monomers other than the above acrylicacid (salt) if necessary. The monomers other than acrylic acid (salt)are not especially limited, but specified examples of them include:anionic unsaturated monomers, such as methacrylic acid, maleic acid,vinylsulfonic acid, styrenesulfonic acid,2-(meth)acrylamido-2-methylpropanesulfonic acid,2-(meth)acryloylethanesulfonic acid, and 2-(meth)acryloylpropanesulfonicacid, and their salts; nonionic unsaturated monomers containing ahydrophilic group, such as acrylamide, meth acrylamide,N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylamide,N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide,2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycolmono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone,N-acryloylpiperidine, and N-acryloylpyrrolidine; cationic unsaturatedmonomers such as N,N-dimethylaminoethyl (meth)acrylate,N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, N,N-dimethylaminopropyl(meth)acrylamide, and their quaternarysalts. These monomers may be used either alone respectively or incombinations with each other.

[0120] In the present invention, when the monomers other than acrylicacid (salt) are used, the ratio of them is preferably 30 mol % or below,more preferably 10 mol % or below, of the total with acrylic acid andits salt. If the above monomers other than acrylic acid (salt) are usedin the above ratio, then the water-absorption properties of theresultant water-absorbent resin are still more improved, and thewater-absorbent resin is obtainable at a still lower cost.

[0121] When the above monomer is polymerized to obtain thewater-absorbent resin as used in the present invention, bulkpolymerization and precipitation polymerization can be carried out.However, considering the performance or the easiness of thepolymerization control, it is preferable to carry out aqueous solutionpolymerization or reversed-phase suspension polymerization using themonomer in the form of its aqueous solution. Incidentally, when themonomer is used in the form of its aqueous solution, the concentrationof the monomer in its aqueous solution (hereinafter referred to as“aqueous monomer solution”) is not especially limited, but is preferablyin the range of 10˜70 weight %, more preferably 20˜40 weight %. Inaddition, when the above aqueous solution polymerization orreversed-phase suspension polymerization is carried out, a solvent otherthan water may be jointly used if necessary, and the kind of the solventas jointly used is not especially limited.

[0122] When the above polymerization is initiated, the following radicalpolymerization initiators, for example, can be used: potassiumpersulfate, ammonium persulfate, sodium persulfate, t-butylhydroperoxide, hydrogen peroxide, and 2,2′-azobis(2-aminodipropane)dihydrochloride.

[0123] Furthermore, a redox initiator is also available by further usinga reductant to promote decomposition of the above polymerizationinitiator and combining both with each other. Examples of the abovereductant include: (bi)sulfurous acid salts such as sodium sulfite andsodium hydrogensulfite; L-ascorbic acid (or its salts); reducible metals(or their salts) such as ferrous salts; and amines. However, thereductant is not especially limited to them.

[0124] The amount of the above polymerization initiator as used isusually in the range of 0.001˜2 mol %, preferably 0.01˜0.1 mol %. In thecase where the amount of the polymerization initiator is less than 0.001mol %, there are disadvantages in that a large amount of monomers remainunreacted, so the amount of monomers, remaining in the resultantwater-absorbent resin, increases. On the other hand, in the case wherethe amount of the polymerization initiator exceeds 2 mol %, there mightbe disadvantages in that the water-soluble content in the resultantwater-absorbent resin increases.

[0125] In addition, the polymerization reaction may be initiated byirradiating the reaction system with active energy rays, such asradiations, electron beam, and ultraviolet rays, instead of using thepolymerization initiators. Incidentally, the reaction temperature in theabove polymerization reaction is not especially limited, but ispreferably in the range of 20-90° C. In addition, the reaction time isnot especially limited either and may fitly be set according to factorssuch as the respective kinds of the monomers and polymerizationinitiators and the reaction temperature.

[0126] The water-absorbent resin, used in the present invention, may bea self-crosslinking type using no crosslinking agent, but preferableones are those which are copolymerized or reacted with aninternal-crosslinking agent having 2 or more polymerizable unsaturatedgroups or 2 or more reactive groups per molecule.

[0127] Specified examples of the above internal-crosslinking agentinclude: N,N-methylenebis(meth)acrylamide, (poly)ethylene glycol(meth)acrylate, (poly)propylene glycol di(meth )acrylate,trimethylolpropane tri(meth)acrylate, glycerol tri(meth)acrylate,glycerol acrylate methacrylate, ethylene-oxide-denaturedtrimethylolpropane tri(meth)acrylate, pentaerythritolhexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallylphosphate, triallylamine, poly(meth)allyloxyalkanes, (poly)ethyleneglycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol,polyethylene glycol, propylene glycol, glycerol, pentaerythritol,ethylenediamine, ethylene carbonate, propylene carbonate,polyethylenimine, and glycidyl (meth)acrylate.

[0128] These internal-crosslinking agents may be used either alonerespectively or in combinations with each other. In addition, theseinternal-crosslinking agents may be added to the reaction system eitherall at once or divisionally. When 2 or more kinds ofinternal-crosslinking agents are used, it is preferable to essentiallyuse a compound with 2 or more polymerizable unsaturated groups,considering the absorption properties of the resultant water-absorbentresin. The use of the internal-crosslinking agent enables the solublecontents to be inhibited from eluting from inside the swollen gel whenthe swollen gel is exposed to a deteriorating condition.

[0129] The amount of the above internal-crosslinking agent as used ispreferably in the range of 0.005˜2 mol %, more preferably 0.02˜0.5 mol%, still more preferably 0.03˜0.3 mol %, of the above hydrophilicmonomers. In the respective cases where the amount of theinternal-crosslinking agent is smaller than 0.005 mol % and where theamount of the internal-crosslinking agent exceeds 2 mol %, thewater-absorbent resin with the static or dynamic deteriorationabsorption capacity, deterioration absorption index, or substantialabsorption capacity at a desired level under a load or thewater-absorbent resin displaying excellent urine resistance might not beobtained.

[0130] When the crosslinking structure is introduced into the internalportion of the water-absorbent resin using the aboveinternal-crosslinking agent, the internal-crosslinking agent may beadded to the reaction system during or after polymerization, or afterpolymerization and neutralization, of the above hydrophilic monomers.

[0131] Incidentally, in the above polymerization, the followingmaterials may be added to the reaction system: various foaming agentssuch as carbonates (or hydrogencarbonates), carbon dioxide, azocompounds, and inert organic solvents; hydrophilic polymers such asstarch-cellulose, derivatives thereof, polyvinyl alcohol, polyacrylicacid (or its salts), and crosslinked polymers of polyacrylic acid (orits salts); various surface-active agents; and chain transfer agentssuch as hypophosphorous acid (or its salts).

[0132] When the water-absorbent resin as obtained by the abovepolymerization reaction is a gel, the above water-absorbent resin isusually dried and, if necessary, pulverized.

[0133] The water content (on the wet basis) of the water-absorbentresin, usable in the present invention, is not especially limited, butis preferably in the range of 1˜40% (but not including 40%), morepreferably 1˜20% still more preferably 1˜10%. In addition, the particlediameter of the water-absorbent resin, usable in the present invention,is usually in the range of 10˜1,000 μm, preferably 50˜800 μm, morepreferably 75˜600 μm (but not including 75 μm), particularly preferably150˜500 μm (but not including 150 μm), on average. The particle shape ofthe water-absorbent resin as obtained in this way, for example, may bespherical, pulverized, or irregular, and is not especially limited, butthose which have the irregular pulverized shapes, as obtained via thepulverization step, are preferably used.

[0134] As to the water-absorbent resin as obtained by the abovepolymerization, drying, and pulverization steps beforesurface-crosslinking, it is preferable to use those which display anabsorption capacity value of 30 g/g or more, preferably 35 g/g or more,under no load, because the effects of the present invention areremarkably shown by such a resin. Of course, the above absorptioncapacity is fitly adjusted according to the purpose.

Addition of Amino Polycarboxylic Acid

[0135] The present invention water-absorbing agent with theaforementioned parameters is, for example, obtainable by mixing thebelow-mentioned specific amino polycarboxylic acid and asurface-crosslinking agent with the above-obtained water-absorbent resinstanding before surface-crosslinking, and by crosslinking the resin,wherein the surface-crosslinking agent is reactable upon the carboxylgroup of the water-absorbent resin.

[0136] The specific amino polycarboxylic acid, usable in the presentinvention, is an amino carboxylic acid with 3 or more carboxyl groups orits salt. Such an amino polycarboxylic acid has high ion blocking orchelating ability to Fe or Cu, and its stability constant to Fe ion ispreferably at least 10, more preferably at least 20. Examples thereofare specified as follows: diethylenetriaminepentaacetate,triethylenetetraaminehexaacetate, cyclohexane-1,2-diaminetetraacetate,N-hydroxyethylethylenediaminetriacetate, ethylene glycol diethyl etherdiaminetetraacetate, ethylenediaminetetrapropionate,N-alkyl-N′-carboxymethyl aspartate, N-alkenyl-N′-carboxymethylaspartate, and their alkaline metal salts, alkaline earth metal salts,ammonium salts, and amine salts. Among these,diethylenetriaminepentaacetate, triethylenetetraaminehexaacetate,N-hydroxyethylethylenediaminetriacetate, and their salts are mostpreferable because they have bulky structures or conformations.

[0137] The amount of the above specific amino polycarboxylic acid asused is different according to the surface-crosslinking agent as usedfor crosslinking of the neighborhood of the surface, but is usually inthe range of 0.00001˜10 weight parts, preferably 0.0001˜1 weight part,per 100 weight parts of the solid content of the water-absorbent resin.In the case where the amount exceeds 10 weight parts, the effectcorresponding to the use is not obtained, and not only is thisuneconomical, but also there are problems in that the absorption amountfalls. In addition, in the case where the amount is smaller than 0.00001weight part, the static deterioration- or substantial absorptioncapacity under a load is hardly raised.

[0138] Examples of the surface-crosslinking agent, usable in the presentinvention, include: polyhydric alcohol compounds such as ethyleneglycol, diethylene glycol, propylene glycol, triethylene glycol,tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropyleneglycol, 2,2,4-trimethyl- 1,3-pentanediol, polypropylene glycol,glycerol, polyglycerol, 2-butene-1,4-diol, 1,3-butanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethyloipropane,diethanolamine, triethanolamine, polyoxypropylene,oxyethylene-oxypropylene block copolymer, pentaerythritol and sorbitol;epoxy compounds such as ethylene glycol diglycidyl ether, polyethylenediglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidylether, polyglycerol polyglycidyl ether, propylene glycol diglycidylether, polypropylene glycol diglycidyl ether and glycidol; polyaminecompounds, such as ethylenediamine, diethylenetriamine,triethylenetetraamine, tetraethylenepentaamine, pentaethylenetetraamineand polyethylenimine, and their inorganic or organic salts (for example,azetidinium salts); polyisocyanate compounds such as 2,4-tolyleneduisocyanate and hexamethylene duisocyanate; polyoxazoline compoundssuch as 1,2-ethylenebisoxazoline; alkylene carbonate compounds such as1,3-dioxolan-2-one, 4-methyl-1,3-dioxolan-2-one,4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one,4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one,1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one,4,6-dimethyl-1,3-dioxan-2-one and 1,3-dioxopan-2-one; haloepoxycompounds, such as epichlorohydrin, epibromohydrin andα-methylepichlorohydrin, and their polyamine adducts (for example,Kymene made by Hercules: registered trademark); silane coupling agentssuch as γ-glycidoxypropyltrimethoxysilane andγ-aminopropyltriethoxysilane; and polyvalent metallic compounds such ashydroxides and chlorides of zinc, calcium, magnesium, aluminum, iron andzirconium. Particularly, the polyhydric alcohols and the alkylenecarbonate compounds are preferable considering the safety in the casewhere a portion of the surface-crosslinking agent remains unreacted.

[0139] The above-exemplified surface-crosslinking agents may be usedeither alone respectively or in combinations with each other. When twoor more surface-crosslinking agents are used jointly with each other, awater-absorbing agent with still more excellent absorption properties isobtainable by combining a first and a second surface-crosslinking agentwhich have solubility parameters (SP values) different from each other.Incidentally, the above-mentioned solubility parameter is a value ascommonly used as a factor showing the polarity of compounds.

[0140] The above-mentioned first surface-crosslinking agent is acompound which is reactive upon a carboxyl group of the water-absorbentresin and has a solubility parameter of 12.5 (cal/cm³)^(½) or more.Examples of the first surface-crosslinking agent include ethyleneglycol, propylene glycol, glycerol, ethylene carbonate, and propylenecarbonate. The above-mentioned second surface-crosslinking agent is acompound which is reactive upon a carboxyl group of the water-absorbentresin and has a solubility parameter less than 12.5 (cal/cm³)^(½).Examples of the second surface-crosslinking agent include glycerolpolyglycidyl ether, (poly)glycerol polyglycidyl ether, ethylene glycoldiglycidyl ether, 1,3-propanediol, trimethylolpropane, 1,3-propanediol,1,6-hexanediol, diethylene glycol, triethylene glycol, tetraethyleneglycol, and 1,4-butanediol.

[0141] The ratio of the surface-crosslinking agent, as used, to thewater-absorbent resin depends on factors such as combinations of thewater-absorbent resin and the surface-crosslinking agent, but is usuallyin the range of 0.005˜10 weight parts, preferably 0.05˜5 weight parts,per 100 weight parts of the water-absorbent resin standing in a drystate. If the surface-crosslinking agent is used in the above range, thewater absorption properties to body fluids (aqueous liquids) such asurine, sweat and menstrual blood can be still more improved. In the casewhere the amount of the surface-crosslinking agent as used is smallerthan 0.005 weight part, the crosslinking density in the neighborhood ofthe surface of the water-absorbent resin can hardly be raised, and thestatic or dynamic deterioration absorption capacity, deteriorationabsorption index, or substantial absorption capacity under a load mightnot be improved. In addition, in the case where the amount of thesurface-crosslinking agent as used exceeds 10 weight parts, thesurface-crosslinking agent is excessive, and this is uneconomical, andfurther, it might be difficult to control the crosslinking density to aproper value, so the static or dynamic deterioration absorptioncapacity, deterioration absorption index, or substantial absorptioncapacity under a load might not be improved.

[0142] In the present invention, it is preferable to use water when thewater-absorbent resin is mixed with the specific amino polycarboxylicacid and the surface-crosslinking agent. The amount of water, as used inthe present invention, is different according to the kind, particlesize, or water content of the water-absorbent resin, but is usually inthe range of 0.5˜10 weight parts, preferably 0.5˜3 weight parts, per 100weight parts of the solid content of the water-absorbent resin. In thecase where the amount of water as used exceeds 10 weight parts, theabsorption capacity might fall. In the case where the amount is smallerthan 0.5 weight parts, it might be difficult to fix the specific aminopolycarboxylic acid onto the surface of the water-absorbent resin, sothe static or dynamic deterioration absorption capacity, deteriorationabsorption index, or substantial absorption capacity under a load couldnot be improved.

[0143] In addition, in the present invention, a hydrophilic organicsolvent may be used when the water-absorbent resin is mixed with thespecific amino polycarboxylic acid and the surface-crosslinking agent.Examples of the usable hydrophilic organic solvent include: alcoholssuch as methyl alcohol, ethyl alcohol, propyl alcohol, isopropylalcohol, butyl alcohol, isobutyl alcohol, t-butyl alcohol, and propyleneglycol; ketones such as acetone; ethers such as dioxane,alkoxy(poly)ethylene glycol, and tetrahydrofuran; amides such asN,N-dimethylformamide; and sulfoxides such as dimethyl sulfoxide. Theamount of the hydrophilic organic solvent as used is different accordingto the kind or particle size of the water-absorbent resin, but isusually in the range of 0˜10 weight parts, preferably 0.1˜5 weightparts, per 100 weight parts of the water-absorbent resin.

[0144] In the present invention, the mixing of the water-absorbent resinwith the specific amino polycarboxylic acid and the surface-crosslinkingagent may be carried out in a state where the water-absorbent resin isdispersed in organic solvents such as cyclohexane and pentane. However,the below processes (1)˜(5), for example, can preferably be exemplifiedas means to display the features of the present invention at maximum.

[0145] (1) A process including the steps of: mixing the specific aminopolycarboxylic acid and the surface-crosslinking agent together,including water and/or the hydrophilic organic solvent if necessary; andthen spraying or dropping the resultant mixture to the water-absorbentresin, thereby mixing them.

[0146] (2) A process including the steps of: mixing the water-absorbentresin with the specific amino polycarboxylic acid or its aqueoussolution; and then spraying or dropping the surface-crosslinking agent,including water and/or the hydrophilic organic solvent if necessary, tothe resultant mixture.

[0147] (3) A process including the steps of: spraying or dropping thesurface-crosslinking agent, including water and/or the hydrophilicorganic solvent if necessary, to the water-absorbent resin, therebymixing them; and then mixing the resultant mixture with the specificamino polycarboxylic acid or its aqueous solution.

[0148] (4) A process including the step of: spraying or dropping thesurface-crosslinking agent and the specific amino polycarboxylic acid,including water and/or the hydrophilic organic solvent if necessary, tothe water-absorbent resin at the same time using means such as twonozzles.

[0149] (5) A process including the steps of: adding the specific aminopolycarboxylic acid to the hydrogel of the water-absorbent resin; thendrying or dehydrating the hydrogel; and spraying or mixing the resultantdried or dehydrated product with the surface-crosslinking agent thatincludes water and/or the hydrophilic organic solvent if necessary (inthis process, the amino polycarboxylic acid can further be added on theway of the step of drying or dehydrating the hydrogel.

[0150] In addition, as is aforementioned, when the specific aminopolycarboxylic acid and the surface-crosslinking agent are mixed withthe water-absorbent resin, it is preferable to mix a solution thereof asprepared using water or the hydrophilic organic solvent. If the specificamino polycarboxylic acid and the water-absorbent resin are mixedtogether in the presence of water, then the value of the staticdeterioration absorption capacity or substantial absorption capacityunder a load can be improved. Incidentally, when water is used formixing, a water-insoluble fine particle powders or a surface-activeagent may be allowed to coexist.

[0151] The mixing apparatus favorable for the above mixing needs to beable to generate a great mixing force to ensure the uniform mixing.Preferable examples of the mixing apparatus, usable in the presentinvention, include: cylinder type mixers, double-wall cone type mixers,high-speed agitation type mixers, V-character-shaped mixers, ribbon typemixers, screw type mixers, fluidized-furnace rotary disk type mixers,gas current type mixers, double-arm type kneaders, internal mixers,pulverizing type kneaders, rotary mixers, and screw type extruders.

[0152] In the present invention, the specific amino polycarboxylic acidand the surface-crosslinking agent are mixed with the water-absorbentresin (preferably, the specific amino polycarboxylic acid and thesurface-crosslinking agent are mixed together and then added to thewater-absorbent resin), and the neighborhood of the surface of thewater-absorbent is then crosslinked by further carrying out the heattreatment.

[0153] When the heat treatment is carried out in the present invention,it is preferable that the treatment temperature is in the range of80˜250° C. The heating temperature lower than 80° C. might not onlylengthen the heating treatment time and therefore deteriorate theproductivity, but also hinder the uniform crosslinking from beingachieved and therefore disable the production of a water-absorbing agentwith excellent static deterioration absorption capacity under a loadwhich is an object of the present invention. In addition, in the casewhere the treatment temperature is higher than 250° C., thewater-absorbent resin might be damaged, so it might be difficult toobtain what is excellent in the static or dynamic deteriorationabsorption capacity, deterioration absorption index, or substantialabsorption capacity under a load.

[0154] The heating treatment can be carried out using conventionaldryers or heating-furnaces, and examples thereof include: channel typemixing dryers, rotary dryers, desk dryers, fluidized-bed dryers,gas-stream type dryers, and infrared dryers.

[0155] In addition, another production process for a water-absorbingagent, according to the present invention, comprises the step of addingthe above specific amino polycarboxylic acid to a surface-crosslinkedwater-absorbent resin having an absorption capacity of 23 (g/g) or moreunder a load.

[0156] The surface-crosslinked water-absorbent resin, as used in thiscase, is generally obtained by mixing the water-absorbent resin, asstands before surface-crosslinking and is obtained in the above way,with the above surface-crosslinking agent, thereby crosslinking theresin.

[0157] This surface-crosslinked water-absorbent resin needs to have anabsorption capacity of 23 (g/g) or more under a load. In the case wherethe absorption capacity under a load is less than 23 (g/g), the staticdeterioration absorption capacity under a load does not fall in therange of the present invention, or the respective absolute values of thestatic and dynamic deterioration absorption capacities and substantialabsorption capacity under a load are low, so the water absorbency cannotsufficiently be performed in diapers even if they are considered to beused for a long time. The absorption capacity under a load is preferablyat least 24 (g/g), more preferably at least 25 (g/g).

[0158] In the present invention, the above specific amino polycarboxylicacid is added to the surface-crosslinked water-absorbent resin having anabsorption capacity of 23 (g/g) or more under a load, but the followingprocess is preferable: an aqueous solution of the specific aminopolycarboxylic acid is prepared, and particles of the water-absorbentresin are combined with each other using water as a binder, therebygranulating the resin. The granulation enlarges the average particlediameter of the water-absorbent resin, and improves the hygroscopicfluidity of the resin and therefore facilitates the handling of theresin. The amount of water as added is usually in the range of 0.1˜20weight parts, preferably 0.1˜10 weight parts, more preferably 0.5˜4weight parts, per 100 weight parts of the water-absorbent resin.

[0159] The process for adding the specific amino polycarboxylic acid andwater is not especially limited, and examples thereof include: a processin which the aqueous solution of the specific amino polycarboxylic acidis added to the water-absorbent resin, thereby granulating the resin;and a process in which the specific amino polycarboxylic acid is addedto the water-absorbent resin, and thereafter water is added to theresin, thereby granulating the resin. A hydrophilic organic solvent,such as methanol, ethanol, isopropyl alcohol, or propylene glycol, canfurther be used to improve the mixability of the specific aminopolycarboxylic acid and water with the water-absorbent resin.Furthermore, a surface-active agent or inorganic fine particles, such assilica or titanium oxide, can be added beforehand or simultaneously.

[0160] The addition of the ion blocking agent (or chelating agent) isnot limited to the aforementioned processes. As is aforementioned, theion blocking agent (or chelating agent) selected from specific aminocarboxylic acids can be fixed onto the surface of the water-absorbentresin by mixing the amino polycarboxylic acid and thesurface-crosslinking agent with a water-absorbent resin beforesurface-crosslinking the water-absorbent resin, thussurface-crosslinking the water-absorbent resin, or by adding the aminopolycarboxylic acid and water to with a specific surface-crosslinkedwater-absorbent resin, thus granulating this resin. Because thedeterioration of water-absorbent resins occurs from their surfaces, itis preferable that the ion blocking agent (or chelating agent) is put inthe neighborhood of the surface of the water-absorbent resin. The ionblocking agent (or chelating agent) can be added when the water-solublemonomer to form the water-absorbent resin is polymerized. However, inthe case where the polymerization of the above monomer is carried out inthe presence of the ion blocking agent (or chelating agent), thepolymerization of the monomer might be hindered by the ion blockingagent (or chelating agent) and therefore might not give thewater-absorbent resin with excellent absorbency, and further, the ionblocking agent (or chelating agent) might lose its ion blocking orchelating ability.

[0161] The water-absorbing agent, as obtained in the above way, is awater-absorbing agent having excellent properties, as have never beenobtained, in that the value of the absorption capacity under no load andthe value of the static or dynamic deterioration absorption capacity,deterioration absorption index, or substantial absorption capacity undera load are excellent. Such a water-absorbing agent is favorably usedeven for diapers which have high resin concentration and low pulpconcentration accompanying the thinning of the diapers in recent years,and this agent further can reduce the leakage in practical use.

[0162] In one of the present invention production processes, forexample, the ion blocking agent and the surface-crosslinking agent whichis reactable upon a carboxyl group are mixed with the above-obtainedwater-absorbent resin having a carboxyl group, whereby thewater-absorbing agent with excellent urine resistance can be obtained.

[0163] Examples of the ion blocking agent, as used in the presentinvention, include the following compounds:

[0164] (1) aminocarboxylic acids and their salts; (2)monoalkylcitramides, monoalkenylcitramides, and their salts; (3)monoalkylmalonamides, monoalkenylmalonamides, and their salts; (4)monoalkylphosphoric esters, monoalkenylphosphoric esters, and theirsalts; (5) N-acylated glutamic acids, N-acylated aspartic acids, andtheir salts; (6) β-diketone derivatives; (7) tropolone derivatives; and(8) organic phosphoric acid compounds.

[0165] As to (1) aminocarboxylic acids and their salts, those which haveat least three carboxyl groups are preferable in respect to their ionblocking ability. Specified examples thereof include: nitrilotriaceticacid, ethylenediaminetetraacetic acid, diethylenetriaminepentaaceticacid, triethylenetetraaminehexaacetic acid,cyclohexane-1,2-diaminetetraacetic acid,N-hydroxyethylethylenediaminetriacetic acid, ethylene glycol diethylether diaminetetraacetic acid, ethylenediaminetetrapropionic acid,N-alkyl-N′-carboxymethylaspartic acid,N-alkenyl-N′-carboxymethylaspartic acid, and their alkaline metal salts,alkaline earth metal salts, ammonium salts, and amine salts.

[0166] (2) Monoalkylcitramides, monoalkenylcitramides, and their saltsare, for example, obtained by dehydration condensation of alcohols withcitric acid.

[0167] (3) Monoalkylmalonamides, monoalkenylmalonamides, and their saltsare, for example, obtained by adding α-olefins to methyl malonate andthen hydrolyzing the resultant adducts.

[0168] Examples of (4) monoalkylphosphoric esters, monoalkenylphosphoricesters, and their salts include laurylphosphoric acid andstearylphosphoric acid.

[0169] Examples of (5) N-acylated glutamic acids, N-acylated asparticacids, and their salts include Amisoft HS-11 and GS-11 as arecommercially available from Ajinomoto Co., Ltd.

[0170] Examples of (6) β-diketone derivatives include acetylacetone andbenzoylacetone.

[0171] Examples of (7) tropolone derivatives include tropolone,β-thujaplicin, and y-thujaplicin.

[0172] Examples of (8) organic phosphoric acid compounds includeethylidenephophonic acid, 1-hydroxyethylidene-1,1-diphophonic acid,aminotrimethylenephophonic acid, ethylenediaminetetra(methylenephophonicacid), and diethylenetriaminepenta(methylenephophonic acid).Particularly, 1-hydroxyethyidene-1,1-diphophonic acid,ethylenediaminetetra(methylenephophonic acid), anddiethylenetriaminepenta(methylenephophonic acid). Preferable examples ofsalts of the organic phosphoric acid compounds include salts of alkalinemetals such as Na and K, and ammonium salts, and amine salts. The aboveorganic phosphoric acid compound is known as one of metal blockingagents.

[0173] Preferable ones among the above ion blocking agents areaminocarboxylic acids having at least three carboxyl groups and theirsalts, and particularly, the most preferable ones arediethylenetriaminepentaacetic acid, triethylenetetraaminehexaaceticacid, cyclohexane-1,2-diaminotetraacetic acid,N-hydroxyethylethylenediaminetriacetic acid, and their salts in respectto the urine resistance because they have bulky structures orconformations.

[0174] The amount of the ion blocking agent, as used in the presentinvention, is different according to the surface-crosslinking agent asused for crosslinking the neighborhood of the surface, but the amount isusually in the range of 0.0001˜10 weight parts, preferably 0.0002˜5weight parts, per 100 weight parts of the solid content of thewater-absorbent resin. In the case where the amount exceeds 10 weightparts, there are problems in that: no effect rewarding the amount isobtained—this is uneconomical—, and further, the absorption amountfalls. In addition, in the case where the amount is smaller than 0.0001weight parts, no effect of improving the urine resistance is obtained.

[0175] The ratio of the surface-crosslinking agent, as used, to thewater-absorbent resin depends on factors such as combinations of thewater-absorbent resin and the surface-crosslinking agent, but is usuallyin the range of 0.01˜10 weight parts, preferably 0.05˜3 weight parts,per 100 weight parts of the water-absorbent resin standing in a drystate. If the surface-crosslinking agent is used in the above range, thewater absorption properties to body fluids (aqueous liquids) such asurine, sweat and menstrual blood can be still more improved. In the casewhere the amount of the surface-crosslinking agent as used is smallerthan 0.01 weight part, the crosslinking density in the neighborhood ofthe surface of the water-absorbent resin can hardly be raised. Inaddition, in the case where the amount of the surface-crosslinking agentas used exceeds 5 weight parts, the surface-crosslinking agent isexcessive, and this is uneconomical, and further, it might be difficultto control the crosslinking density to a proper value.

[0176] In the present invention, it is preferable to use water when thewater-absorbent resin is mixed with the ion blocking agent and thesurface-crosslinking agent. The amount of water, as used in the presentinvention, is different according to the kind, particle size, or watercontent of the water-absorbent resin, but is usually in the range of0.5˜10 weight parts, preferably 0.5˜3 weight parts, per 100 weight partsof the solid content of the water-absorbent resin. In the case where theamount of water as used exceeds 10 weight %, the absorption capacitymight fall. In the case where the amount is smaller than 0.5 weight %,it might be difficult to fix the ion blocking agent onto the surface ofthe water-absorbent resin, so the urine resistance could not beimproved.

[0177] All the aforementioned modes for carrying out the mixing andadding of the amino polycarboxylic acid, with no modification but theabove-mentioned respects, can be applied to the specific modes forcarrying out the mixing of the ion blocking agent and thesurface-crosslinking agent with the water-absorbent resin in theproduction process as mentioned immediately above.

[0178] If the neighborhood of the surface of the water-absorbent iscrosslinked in the above way, the soluble contents can be prevented fromeluting from inside the water-absorbent resin. However, when absorbingurine and so on containing L-ascorbic acid, the water-absorbent resindeteriorates with time because its main chain and crosslinking structureare cut by actions of L-ascorbic acid and a very small amount of heavymetal ions, such as iron or copper, as mingle in the production processfor the water-absorbent resin or diapers or are contained in the urine.Especially, the neighborhood of the surface of the water-absorbent iseasily deteriorated, so the elution of the soluble contents cannot besuppressed. Therefore, the absorbency of the water-absorbent resin fallswith time when the resin absorbs urine. In the present invention, thesurface-crosslinking agent and the ion blocking agent are mixed with thewater-absorbent resin, whereby the deterioration of the water-absorbentresin, especially, the deterioration of its surface neighborhood, isprevented to suppress the elution of the soluble contents.

[0179] In another production process of the present invention, water andthe aforementioned ion blocking agent are added (e.g. by spraying) tothe water-absorbent resin (as has beforehand been surface-crosslinkedwith the aforementioned surface-crosslinking agent) to bind particles ofthe water-absorbent resin to each other using water as the binder, thusmaking a granulation to give the water-absorbing agent with excellenturine resistance. The aforementioned water-absorbent resin is obtainableby crosslinking the neighborhood of the surface of a water-absorbentresin as obtained by polymerizing a monomer, which needs to include anunsaturated carboxylic acid, in the presence of an internal-crosslinkingagent. The use of the internal-crosslinking agent can prevent solublecontents from eluting from inside the swollen gel of the resin when theswollen gel is exposed to a condition having a property to deterioratethe gel. The granulation enlarges the average particle diameter of thewater-absorbent resin and improves the hygroscopic fluidity of theresin, thereby facilitating its handling. The amount of water, as added,is in the range of usually 0.1˜20 weight %, preferably 0.1˜10 weight %,more preferably 0.5˜4 weight %, per 100 weight parts of thewater-absorbent resin. In the case where the amount of water is smallerthan 0.1 weight %, it is difficult to granulate the water-absorbentresin particles, and further, the ion blocking agent cannot be fixed tothe neighborhood of the surface of the water-absorbent resin. Inaddition, in the case where the amount of water is larger than 20 weight%, the water-absorbent resin swells up to its inside to form a gel, sothere is a possibility that the granulation product, as aimed in thepresent invention, could not be obtained, and that the crosslinked layerof the surface of the water-absorbent resin might be destroyed. In thisprocess, the above water-absorbent resin, as has beforehand beensurface-crosslinked, is recommended to have an absorption capacity ofusually at least 20 (g/g), preferably at least 22 (g/g), more preferablyat least 24 (g/g), for a 0.9 wt % aqueous sodium chloride solution(physiological sodium chloride solution) under a load of 0.7 psi,because in the case where the absorption capacity under the load islower than 20 (g/g), there is a possibility that the water absorbencycould not sufficiently be performed in diapers.

[0180] The granulation method involving the addition of the ion blockingagent is not especially limited, but examples thereof other than theabove-mentioned ones include a method in which the ion blocking agent isadded to the water-absorbent resin, and then water is added, thusgranulating the resin. Hydrophilic organic solvents such as methanol,ethanol, and isopropyl alcohol can jointly be used for the purpose ofimproving the miscibility of the ion blocking agent, water, and thewater-absorbent resin. Furthermore,s surface-active agents and inorganicfine particles such as silica and titanium oxide can be added beforehandor at the same time.

[0181] In the present invention, the water-absorbent resin withexcellent resistance can further be obtained by adding a chelating agentof a specific structure while and/or after the water-absorbent resin ispolymerized in the aforementioned way.

[0182] The chelating agent of a specific structure, usable in thepresent invention, is one or two or more compounds selected from thegroup consisting of compounds of general formulae (1) and (2) below andmaleic hydrophilic polymers (including salts) (3),

[0183] wherein general formula (1) is:

[0184] wherein: n, X¹, and R¹˜R³ denote the following numbers andstructures:

[0185] and wherein general formula (2) is:

[0186] wherein: m, X², and R⁵˜R⁸ denote the following numbers andstructures:

[0187] Examples of the chelating agent of general formula (1) aboveinclude: N-carboxymethyl-aspartic acid, N,N-dicarboxymethyl-asparticacid, N-carboxyethyl-aspartic acid, N,N-dicarboxyethyl-aspartic acid,N-(1,2-dicarboxyethyl)-aspartic acid,N-(1,2-dicarboxy-2-hydroxyethyl)-aspartic acid,N-carboxymethyl-2-hydroxy-aspartic acid,N,N-dicarboxymethyl-2-hydroxy-aspartic acid,N-carboxyethyl-2-hydroxy-aspartic acid,N-(1,2-dicarboxyethyl)-2-hydroxy-aspartic acid, N-carboxymethyl-glutamicacid, N,N-dicarboxymethyl-glutamic acid, N-carboxyethyl-glutamic acid,N,N-dicarboxyethyl-glutamic acid, N-(1,2-dicarboxyethyl)-glutamic acid,N-(1,2-dicarboxy-2-hydroxyethyl)-glutamic acid, and their sodium,potassium and ammonium salts.

[0188] Examples the chelating agent of general formula (2) aboveinclude: N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine,N,N′-bis(1,2-dicarboxy-2-hydroxyethyl)-ethylenediamine,N,N′-bis(1,2-dicarboxyethyl)-N,N′-dicarboxymethylethylenediamine,N,N′-bis(1,2-dicarboxy-2-hydroxyethyl)-N,N′-dicarboxymethylethylenediamine,and their sodium, potassium and ammonium salts.

[0189] In addition, examples of maleic hydrophilic polymers (includingsalts) (3) include:

[0190] hydrophilic polymers which are obtained by polymerizing 1˜100 mol% of maleic acid, fumaric acid, itaconic acid, and their sodium,potassium and ammonium salts with 0˜99 mol % of acrylic acid,methacrylic acid, and their sodium, potassium and ammonium salts andhave an average molecular weight of 500˜1,000,000;

[0191] preferably, hydrophilic polymers which are obtained bypolymerizing 5˜100 mol % of maleic acid and its salts with 0˜95 mol % ofacrylic acid and its salts and have an average molecular weight of1,000˜200,000; and

[0192] more preferably, hydrophilic polymers which are obtained bypolymerizing 10˜50 mol % of maleic acid and its salts with 50˜90 mol %of acrylic acid and its salts and have an average molecular weight of1,000˜100,000.

[0193] Chelating agents of general formulae (1) and (2) are preferableamong the above chelating agents in view of their safety andbiodegradability. The chelating agents of general formulae (1) and (2)can favorably be used in any form of their optical isomers and racemicmodifications. Particularly preferable examples thereof include:N-(1,2-dicarboxy-2-hydroxyethyl)-aspartic acid,N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine,N,N′-bis(1,2-dicarboxy-2-hydroxyethyl)-ethylenediamine, and theirsodium, potassium and ammonium salts.

[0194] The amount of the above chelating agent, as used, is notespecially limited, and is different according to the kind and theaddition method of the chelating agent, but the amount is in the rangeof usually 0.00001˜30 weight parts per 100 weight parts of thewater-absorbent resin.

[0195] Examples of the method of adding the chelating agent to thewater-absorbent resin include:

[0196] (1) (Addition during the polymerization): A method in which theabove chelating agent is added to an aqueous solution of a water-solubleethylenically unsaturated monomer which can form the water-absorbentresin by polymerization. The aqueous monomer solution might containheavy metals which eluted from pipes or reaction vessels or werecontained in raw materials such as caustic soda. In the case where thepolymerization is carried out in the presence of ions of such heavymetals, there is a possibility that water-absorbent resins which easilydeteriorate when swelling might be obtained, or that the swollen gelmight easily be deteriorated by the residual heavy metal ions. However,the polymerization of the above monomer in the presence of the abovechelating agent could give water-absorbent resins with excellentstability of their swollen gels with time.

[0197] It is preferable that the chelating agent is beforehand added tothe aqueous monomer solution to carry out the polymerization. However,the chelating agent may be added after the initiation of thepolymerization. The amount of the chelating agent, as added in thepolymerization step, is in the range of usually 0.00001˜1 weight part,preferably 0.00002˜0.1 weight part, more preferably 0.00005˜0.01 weightpart, per 100 weight parts of the solid content of the monomer. In thecase where the amount of the chelating agent is smaller than 0.00001weight part, the water-absorbent resin with excellent stability of itsswollen gel with time cannot be obtained. In the case where the amountof the chelating agent exceeds 1 weight part, the polymerization of themonomer might be hindered.

[0198] (2) (Addition to polymer gel): A method in which the abovechelating agent is added to a hydrogel as obtained by polymerizing awater-soluble ethylenically unsaturated monomer which can form awater-absorbent resin by polymerization.

[0199] The solid content of the hydrogel is generally in the range of20˜90 weight %. The gel to which the chelating agent was added can bedried by conventional means. The drying temperature is preferably 120°C. or higher.

[0200] The amount of the chelating agent, as added, is in the range ofusually 0.00001˜30 weight parts, preferably 0.00005˜10 weight parts, per100 weight parts of the solid content of the hydrogel. In the case wherethe amount of the chelating agent is smaller than 0.00001 weight part,the water-absorbent resin with excellent stability of its swollen gelwith time cannot be obtained. In the case where the amount of thechelating agent exceeds 30 weight parts, no effects rewarding thisamount might be obtained, or the water absorption capacity mightdecrease rather than increase.

[0201] The hydrogel resultant from the polymerization, for example, thehydrogel as obtained by aqueous solution polymerization, can be dried asit is plate-shaped. However, considering the drying efficiency or theperformance of the water-absorbing agent to obtain, it is generallypreferable to disintegrate or cut the hydrogel into the size of 0.1˜10mm. As to the shape of the hydrogel, various ones can preferably be usedfor the present invention: for example, plate-shaped, squarish,irregular by pulverization, spherical, fibrous, bar-shaped,approximately spherical, scaly. The hydrogel resultant from thepolymerization can be then neutralized with alkalies. In addition, thechelating agent can be added to the hydrogel as obtained byreversed-phase suspension polymerization and suspended in organicsolvents, or can be added in the azeotropic dehydration step.

[0202] The above chelating agent can be added to the hydrogel in anystep before the end of the hydrogel drying. For example, the chelatingagent can be added to the hydrogel as formed in a reaction vessel, orcan be added in the disintegration step of the hydrogel, or can be addedto the disintegrated hydrogel or can be added on the way of drying.

[0203] Specifically, the following methods can be exemplified: a methodin which the chelating agent is added when the hydrogel is disintegratedwith tools such as kneaders or meat choppers; and a method in which thechelating agent is added in the neighborhood of inlets of dryers. Thechelating agent can be used in a state where it is powdery or dissolvedor dispersed in water or other solvents. In addition, the chelatingagent can be coated or sprayed to the surface of the hydrogel.

[0204] The hydrogel, to which the chelating agent was added, can be, forexample, dried with hot-air dryers, gas stream dryers, fluidized-beddryers, drum dryers, microwaves, and far infrared rays. The dryingtemperature is usually 120° C. or higher, preferably in the range of150˜250° C., more preferably 160˜220° C. In the case where the dryingtemperature is lower than 120° C., the drying needs too long a time, andfurther, the hydrogel is heated in a gelled state for a long time andtherefore easily deteriorates.

[0205] (3) (Addition to water-absorbent resin (case 1)): A method inwhich the above chelating agent is added (e.g. by mixing) to thewater-absorbent resin along with the surface-crosslinking agent havingtwo or more functional groups reactable upon functional groups of thewater-absorbent resin.

[0206] The amount of the above chelating agent, as used in this method,is different according to the surface-crosslinking agent as used tocrosslink the neighborhood of the surface, but the amount of thechelating agent is in the range of usually 0.0001˜10 weight parts,preferably 0.0002˜5 weight parts, per 100 weight parts of the solidcontent of the water-absorbent resin. In the case where the amountexceeds 10 weight parts, there are problems in that: no effect rewardingthis amount is obtained and there are therefore economicaldisadvantages, and further, the absorption amount falls. In addition, inthe case where the amount is smaller than 0.0001 weight part, no effectof improving the urine resistance is obtained.

[0207] As to the surface-crosslinking agent in this method, those whichare above-explained on the addition of the amino polycarboxylic acid canbe used.

[0208] The ratio of the surface-crosslinking agent, as used, to thewater-absorbent resin depends on factors such as combinations of thewater-absorbent resin and the surface-crosslinking agent, but is usuallyin the range of 0.01˜10 weight parts, preferably 0.05˜3 weight parts,per 100 weight parts of the water-absorbent resin standing in a drystate. If the surface-crosslinking agent is used in the above range, thewater absorption properties to body fluids (aqueous liquids) such asurine, sweat and menstrual blood can be still more improved. In the casewhere the amount of the surface-crosslinking agent as used is smallerthan 0.01 weight part, the crosslinking density in the neighborhood ofthe surface of the water-absorbent resin can hardly be raised. Inaddition, in the case where the amount of the surface-crosslinking agentas used exceeds 5 weight parts, the surface-crosslinking agent isexcessive, and this is uneconomical, and further, it might be difficultto control the crosslinking density to a proper value.

[0209] In the present invention, it is preferable to use water when thewater-absorbent resin is mixed with the chelating agent and thesurface-crosslinking agent. The amount of water, as used in the presentinvention, is different according to the kind, particle size, or watercontent of the water-absorbent resin, but is usually in the range of0.5˜10 weight parts, preferably 0.5˜3 weight parts, per 100 weight partsof the solid content of the water-absorbent resin. In the case where theamount of water as used exceeds 10 weight parts, the absorption capacitymight fall. In the case where the amount is smaller than 0.5 weightparts, it might be difficult to fix the chelating agent onto the surfaceof the water-absorbent resin, so the urine resistance could not beimproved.

[0210] All the aforementioned modes for carrying out the mixing andadding of the amino polycarboxylic acid, with no modification but theabove-mentioned respects, can be applied to the specific modes forcarrying out the mixing of the chelating agent and thesurface-crosslinking agent with the water-absorbent resin in this method(3).

[0211] (4) (Addition to water-absorbent resin (case 2)): A method inwhich the above chelating agent is added to the surface-crosslinkedwater-absorbent resin.

[0212] A water-absorbent resin, as favorably used as thesurface-crosslinked water-absorbent resin, has an absorption capacity ofusually at least 20 (g/g), preferably at least 22 (g/g), more preferablyat least 24 (g/g), for a 0.9 wt % aqueous sodium chloride solution(physiological sodium chloride solution) under a load of 0.7 psi. In thecase where the absorption capacity under the load is lower than 20(g/g), there is a possibility that the water absorbency could notsufficiently be performed in diapers.

[0213] The amount of the chelating agent, as used in this method (4), isusually in the range of 0.00001˜10 weight parts, preferably 0.0001˜5weight parts, per 100 weight parts of the solid content of thewater-absorbent resin. In the case where the amount exceeds 10 weightparts, there are problems in that: no effect rewarding the amount isobtained—this is uneconomical—, and further, the absorption amountfalls. In addition, in the case where the amount is smaller than 0.00001weight parts, no effect of improving the urine resistance is obtained.

[0214] Examples of the method for mixing the above surface-crosslinkedwater-absorbent resin and the chelating agent together in this method(4) include: a method in which the water-absorbent resin and thechelating agent are blended together under dry conditions; and a methodin which a mixture of the chelating agent with water, an organicsolvent, or a water-organic solvent mixed solvent is added to thewater-absorbent resin.

[0215] In this method (4), water and the above chelating agent are added(e.g. by spraying) to the surface-crosslinked water-absorbent resin tothereby bind particles of the water-absorbent resin to each other usingwater as the binder, whereby the resin can be granulated. Thegranulation enlarges the average particle diameter of thewater-absorbent resin and improves the hygroscopic fluidity of theresin, thereby facilitating its handling. The amount of water, as added,is in the range of usually 0˜50 weight %, preferably 0.01˜10 weight %,per 100 weight parts of the water-absorbent resin. In the case where theamount of water is smaller than 0.1 weight %, it is difficult togranulate the water-absorbent resin particles, and further, thechelating agent cannot be fixed to the neighborhood of the surface ofthe water-absorbent resin. In addition, in the case where the amount ofwater is larger than 50 weight %, the water-absorbent resin swells up toits inside to form a gel, so there is a possibility that the granulationproduct, as aimed in the present invention, could not be obtained, andthat the crosslinked layer of the surface of the water-absorbent resinmight be destroyed.

[0216] The granulation method involving the addition of the chelatingagent is not especially limited, but examples thereof other than theabove-mentioned ones include a method in which the chelating agent isadded to the water-absorbent resin, and then water is added, thusgranulating the resin. Hydrophilic organic solvents such as methanol,ethanol, and isopropyl alcohol can jointly be used for the purpose ofimproving the miscibility of the chelating agent, water, and thewater-absorbent resin. Furthermore, surface-active agents and inorganicfine particles such as silica and titanium oxide can be added beforehandor at the same time.

[0217] (5) (Addition to water-absorbent resin (case 3)): A method inwhich the above chelating agent is added when fine powders of thewater-absorbent resin are recovered.

[0218] In the production steps for the water-absorbent resin, forexample, a polymer powder to form a water-absorbent resin might beclassified with a screen of the predetermined size, and fine particlesas removed from the water-absorbent resin in this classification mightbe added in any step of producing the water-absorbent resin and therebyrecovered. When this recovery of the fine particles is carried out, thechelating agent can be added.

[0219] As to the water-absorbent resin, either of thesurface-crosslinked one and the not yet surface-crosslinked one can beused. The particle diameter of the water-absorbent resin, as used forthe recovery, is not especially limited, but is generally 300 μm orbelow, preferably 225 μm or below, more preferably 150 μm or below.

[0220] The amount of water, as added, is, for example, in the range ofusually 0.1˜2,000 weight parts, preferably 10˜900 weight parts, per 100weight parts of water-absorbent resin. In the case where the amount ofwater is smaller than 0.1 weight part, the recycling is difficult. Inthe case where the amount is larger than 2,000 weight parts, thedeterioration of the recycled water-absorbent resin cannot be prevented.

[0221] The amount of the chelating agent, as added, is in the range ofusually 0.00001˜30 weight parts, preferably 0.1˜10 weight parts, per 100weight parts of dry water-absorbent resin. In the case where the amountof the chelating agent is smaller than 0.00001 weight part, it isdifficult to obtain the water-absorbing agent which displays excellentgel stability with time. In the case where the amount of the chelatingagent is larger than 30 weight parts, no effect rewarding this amountmight be obtained.

[0222] The chelating agent can be added either in the form of aqueoussolution to the water-absorbent resin, or to the water-absorbent resinas mixed with water. In addition, it is also permissible to blend thechelating agent with the water-absorbent resin under dry conditions andto further mix water with the resultant mixture.

[0223] The recycling of fine powders of the water-absorbent resin in thepresence of the chelating agent in the above way can prevent thewater-absorbent resin from deteriorating in the recycling.

Addition of Other Materials

[0224] If necessary, various functions may be given to the abovewater-absorbing agent by adding thereto the following materials:deodorants, antimicrobial agents, perfumes, various inorganic powders,foaming agents, pigments, dyes, hydrophilic short fibers, plasticizers,pressure sensitive adhesives, surface-active agents, manure, oxidants,reductants, water, and salts.

[0225] Examples of the inorganic powder include inactive substances (forexample, inactive to aqueous liquids) such as fine particles of variousinorganic compounds and clay minerals. It is preferable that the aboveinorganic powder has moderate affinity to water and is insoluble orhardly soluble in water. Specified examples thereof include: metaloxides such as silicon dioxide and titanium oxide; silicic acid (or itssalts) such as natural zeolite and synthetic zeolite; kaolin; talc;clay; and bentonite. Among these, preferable ones are silicon dioxideand silicic acid (or its salts), and more preferable ones are silicondioxide and silicic acid (or its salts) with the average particlediameter of 200 μm or less as measured by the Coulter Counter Method.

[0226] The ratio of the inorganic powder to the water-absorbent resindepends on factors such as combinations of the water-absorbent resinwith the inorganic powder, but is in the range of usually 0.001˜10weight parts, preferably 0.01˜5 weight parts, per 100 weight parts ofthe water-absorbent resin. The method for mixing the water-absorbentresin with the inorganic powder is not especially limited, and dry blendmethods or wet mixing methods are, for example, available, but the dryblend methods are preferable.

Uses of Water-Absorbing Agent

[0227] The water-absorbing agent, as obtained in the above way, isformed into an absorbent article by, for example, compounding(combining) the resin with fibrous materials such as pulp.

[0228] Examples of the absorbent article include: sanitary materials(body-fluid-absorbent articles), such as paper diapers, sanitarynapkins, incontinent pads, wound-protecting materials, and wound-curingmaterials; absorbent articles for urine of pets; materials for civilengineering and architecture, such as water-holding materials,water-cutting-off materials, packing materials, and hydrogel bags, forbuilding materials or soil; articles for foods, such as drip-absorbentmaterials, freshness-keeping materials, and coldness-keeping materials;various industrial articles, such as oil-water-separating materials,dewfall-preventing materials, and solidification materials; andagricultural and horticultural articles, such as water-holding materialsfor plants and soil; but the absorbent article is not especiallylimited. Incidentally, the paper diaper is, for example, formed bylaminating a back sheet of liquid-impermeable material, the abovewater-absorbent composition, and a top sheet of liquid-permeablematerial in this order and fixing them together, and then furnishing theresultant laminate with attachments such as gathers (elastic parts) orso-called tape fasteners. In addition, the paper diaper can includepants with paper diapers as used to train infants for urination andshit-evacuation.

[0229] Hereafter, a detailed explanation is made on the presentinvention absorbent matter, which displays excellent absorptionproperties, as a use of the present invention water-absorbing agenthaving the aforementioned parameters.

[0230] Absorbent Matter

[0231] The above present invention water-absorbing agent is usable inthe form of an absorbent matter. This absorbent matter comprises thewater-absorbing agent and a fibrous base material such as a hydrophilicfiber. The weight ratio of the water-absorbing agent to the total of thewater-absorbing agent and the fibrous base material is 0.4 or more. Inthe case where the absorbent matter, for example, comprises thewater-absorbing agent and the hydrophilic fiber, a constitution of theabsorbent matter comprising a homogeneous mixture of the water-absorbingagent and the hydrophilic fiber is, for example, preferable forsufficiently displaying effects of the present invention. Examples ofsuch a constitution include: a constitution comprising a homogeneousmixture of the water-absorbing agent and the hydrophilic fiber; aconstitution comprising a layer of a homogeneous mixture of thewater-absorbing agent and the hydrophilic fiber and a layer of thehydrophilic fiber as laminated on the preceding layer; a constitutioncomprising a layer of a homogeneous mixture of the water-absorbing agentand the hydrophilic fiber, a layer of the hydrophilic fiber, and thewater-absorbing agent as interposed between these layers; and further aconstitution comprising the water-absorbing agent as interposed betweenlayers of the hydrophilic fiber; and still further a constitutioncomprising a sheet of the water-absorbing agent as shaped by combining aspecific amount of water with the water-absorbing agent. Incidentally,the constitution of the absorbent matter is not limited to theabove-mentioned examples thereof.

[0232] A preferable water-absorbing agent as used for the absorbentmatter is any one of the following: what has an absorption capacity of30 (g/g) or more under no load and static deterioration absorptioncapacity (1) of 20 (g/g) or more under a load, what has an absorptioncapacity of 30 (g/g) or more under no load and a dynamic deteriorationabsorption capacity of 20 (g/g) or more under a load, and what has anabsorption capacity of 30 (g/g) or more under no load and staticdeterioration absorption capacity (4) of 30 (g/g) or more under a load,because these water-absorbing agents can improve the absorptionabilities of the absorbent matter in practical use.

[0233] The reason why it is preferable to use the water-absorbing agenthaving an absorption capacity of 30 (g/g) or more under no load andstatic deterioration absorption capacity (1) of 20 (g/g) or more under aload is as follows. In the case where the absorption capacity under noload is less than 30 (g/g), the absorption abilities are insufficient,and the leakage and so on are apt to occur especially when thewater-absorbing agent is used for absorbent articles including theabsorbent matter and having high resin concentration. The absorptioncapacity under no load is preferably at least 33 (g/g), more preferablyat least 35 (g/g). In addition, in the case where static deteriorationabsorption capacity (1) under a load is less than 20 (g/g), similarly,the absorption abilities of the absorbent article are insufficient, andthe leakage and so on are apt to occur, or the absorption actionsgreatly vary due to factors such as composition changes of liquids to beabsorbed, so the stable absorption properties are not obtainable. Staticdeterioration absorption capacity (1) under a load is preferably 23(g/g) or more.

[0234] The reason why it is preferable to use the water-absorbing agenthaving an absorption capacity of 30 (g/g) or more under no load and adynamic deterioration absorption capacity of 20 (g/g) or more under aload is as follows. In the case where the absorption capacity under noload is less than 30 (g/g), the absorption abilities are insufficient,and the leakage and so on are apt to occur especially when thewater-absorbing agent is used for absorbent articles including theabsorbent matter and having high resin concentration. The absorptioncapacity under no load is preferably at least 33 (g/g), more preferablyat least 35 (g/g). In addition, in the case where the dynamicdeterioration absorption capacity under a load is less than 20 (g/g),similarly, the absorption abilities of the absorbent article areinsufficient, and the leakage and so on are apt to occur, or theabsorption actions greatly vary due to factors such as compositionchanges of liquids to be absorbed and dynamic force as applied to theresin, so the stable absorption properties are not obtainable. Thedynamic deterioration absorption capacity under a load is preferably 23(g/g) or more.

[0235] The reason why it is preferable to use the water-absorbing agenthaving an absorption capacity of 30 (g/g) or more under no load andstatic deterioration absorption capacity (4) of 30 (g/g) or more under aload is as follows. In the case where the absorption capacity under noload is less than 30 (g/g), the absorption abilities are insufficient,and the leakage and so on are apt to occur especially when thewater-absorbing agent is used for absorbent articles including theabsorbent matter and having high resin concentration. The absorptioncapacity under no load is preferably at least 33 (g/g), more preferablyat least 35 (g/g). In addition, in the case where static deteriorationabsorption capacity (4) under a load is less than 30 (g/g), similarly,the absorption abilities of the absorbent article are insufficient, andthe leakage and so on are apt to occur, or the absorption actionsgreatly vary due to factors such as composition changes of liquids to beabsorbed, so the stable absorption properties are not obtainable. Staticdeterioration absorption capacity (4) under a load is preferably atleast 32 (g/g), more preferably at least 34 (g/g).

[0236] Examples of the above-mentioned fibrous base material includehydrophilic fibers such as: cellulose fibers, for example, mechanicalpulp, chemical pulp, semichemical pulp, digested pulp, as obtained fromwood; and artificial cellulose fibers, for example, rayon, acetates.Among the above-exemplified fibers, cellulose fibers are preferable. Inaddition, the hydrophilic fibers may comprise synthetic fibers such aspolyamides, polyesters, and polyolefins. Incidentally, the fibrous basematerial is not limited to the above-exemplified fibers. If formed intoa sheet such as mat or web or into a tape, the fibrous base material caneasily be utilized as the below-mentioned absorbent layer.

[0237] In addition, in the case where the ratio of the fibrous materialsuch as the hydrophilic fiber in the absorbent matter is relativelysmall, the absorbent matters, namely, the hydrophilic fibers, may beallowed to adhere together using adhesive binders. If the hydrophilicfibers are allowed to adhere together, the strength and the shaperetainability of the absorbent matter before or during the use thereofcan be enhanced.

[0238] Examples of the above-mentioned adhesive binders include:heat-sealable fibers such as polyolefin fibers (e.g., polyethylene,polypropylene, ethylene-propylene copolymers, 1-butene-ethylenecopolymers); and adhesive emulsions. These adhesive binders may be usedeither alone respectively or in combinations with each other. The weightratio of the hydrophilic fiber and the adhesive binder is preferably inthe range of 50/50 to 99/1, more preferably 70/30 to 95/5, still morepreferably 80/20 to 95/5.

[0239] It is preferable that the absorbent matter including the abovepresent invention water-absorbing agent satisfies a static deteriorationconcentration absorption index of equation (1) below:

static deterioration concentration absorption index=X(1−α)+Yα≧23  (1)

[0240] wherein: X is the absorption capacity (g/g) under no load of thewater-absorbing agent;

[0241] Y is static deterioration absorption capacity (1) (g/g) under aload of the water-absorbing agent; and

[0242] α is the weight ratio of the water-absorbing agent to the totalof the water-absorbing agent and the fibrous base material (α≧0.4).

[0243] The static deterioration concentration absorption index in thepresent invention is the sum of:

[0244] the product of the absorption capacity under no load, X (g/g), ofthe water-absorbing agent with the weight ratio of the fibrous basematerial in the absorbent matter; and

[0245] the product of static deterioration absorption capacity (1) undera load, Y (g/g), of the water-absorbing agent with the weight ratio ofthe water-absorbing agent in the absorbent matter.

[0246] This static deterioration concentration absorption index is ascale as newly found by the present inventors as the index to predictthe absorption abilities of the absorbent matter in practical use.

[0247] If the weight ratio, α, of the water-absorbing agent to the totalof the water-absorbing agent and the fibrous base material is selectedalong with the water-absorbing agent such that the static deteriorationconcentration absorption index of equation (1) above can be 23 or more,then the absorption amount in a state near practical use of theresultant absorbent matter can be increased. Furthermore, ifwater-absorbing agents, of which the absorption capacity under no load,X (g/g), and static deterioration absorption capacity (1) under a load,Y (g/g), give the same static deterioration concentration absorptionindex value as each other, are selected, then absorbent matters havingalmost the same absorption amount as each other in a state nearpractical use can be produced even if their absorption capacity valuesare different from each other. In addition, as is aforementioned, staticdeterioration absorption capacity (1) under a load in this case is avalue as measured by a specific new evaluation process. As is mentionedabove, there are many prior art documents that disclose the evaluationof the absorption capacity under a load, in which the measurement isgenerally made in a comparatively short period of time using a liquidwith an electrolyte concentration near that of urine. However, in manycases, the actual wearing time of diapers extends for a long time of 6hours or more. Therefore, water-absorbent resins (water-absorbingagents), which provide excellent results with regard to the aboveconventional evaluation items as have been proposed so far, do notnecessarily exhibit excellent performance in practical use as well. Inaddition, urine contains compounds which change (deteriorate) theproperties of the resin with time, and the existence of these compoundsalso largely influences the absorption actions of the water-absorbentresin in practical use. Furthermore, the present inventors haveclarified that the degree of significance for such properties varieswith the weight ratio, α, of the water-absorbing agent to the total ofthe water-absorbing agent and the fibrous base material. That is to say,seeking after only the absorption capacity value under a load could notimprove the absorption amount in a state near practical use of absorbentmatters in such as paper diapers containing fibrous base materials. Forthis improvement, it is necessary to select the resin such that thestatic deterioration concentration absorption index as defined in thepresent invention can satisfy the value of 23 or more.

[0248] As to the absorbent matter of the present invention, as theweight ratio ax of the water-absorbing agent to the total of thewater-absorbing agent and the fibrous base material gets smaller, theabsorption capacity under no load, X, tends to be more important forusable water-absorbing agents, but, considering the static deteriorationconcentration absorption index value, resins having a high value ofstatic deterioration absorption capacity (1) under a load, Y, can alsobe used. In addition, as α gets larger, static deterioration absorptioncapacity (1) under a load, Y, tends to be more important for usablewater-absorbing agents, but, considering the static deteriorationconcentration absorption index value, resins having a high value ofabsorption capacity under no load, X, can be also used. Preferably, whenα is 0.4 or more, the effects of the present invention are greatlyexhibited. More preferably, α is 0.6 or more. In the case where α isless than 0.4, differences of the physical properties of somewater-absorbing agents are not greatly shown as differences of theperformances of absorbent matters.

[0249] In the present invention, the weight ratio, α, of thewater-absorbing agent to the total of the water-absorbing agent and thefibrous base material is determined along with the water-absorbing agentsuch that the value of the static deterioration concentration absorptionindex of equation (1) will be 23 or more. In the case where the staticdeterioration concentration absorption index is less than 23, theabsorption amount in a state near practical use of the absorbent matteris low: for example, in the case of paper diapers including theabsorbent matter, the probability of the occurrence of leakage is high.Preferably, the value of the static deterioration concentrationabsorption index is 26 or more.

[0250] In addition, even if the value of the static deteriorationconcentration absorption index is 23 or more, the amount of thewater-absorbing agent as used is preferably 8 (g) or more. An absorbentarticle, of which the amount of the water-absorbing agent as used issmaller than 8 (g), might lack the dry feeling as a product and displaya very large amount of desorption. The amount of the water-absorbingagent as used is more preferably in the range of 10˜20 (g). In addition,the basis weight of the water-absorbing agent in the absorbent matter ispreferably 100 (g/m²) or more.

[0251] It is also preferable that the absorbent matter including theabove present invention water-absorbing agent satisfies a dynamicdeterioration concentration absorption index of equation (2) below:

dynamic deterioration concentration absorption index=X(1−γ)+Aγ≧23  (2)

[0252] wherein: X is the absorption capacity (g/g) under no load of thewater-absorbing agent;

[0253] A is a dynamic deterioration absorption capacity (g/g) under aload of the water-absorbing agent; and

[0254] γ is the weight ratio of the water-absorbing agent to the totalof the water-absorbing agent and the fibrous base material (γ≧0.4).

[0255] The dynamic deterioration concentration absorption index in thepresent invention is the sum of:

[0256] the product of the absorption capacity under no load, X (g/g), ofthe water-absorbing agent with the weight ratio of the fibrous basematerial in the absorbent matter; and

[0257] the product of the dynamic deterioration absorption capacityunder a load, A (g/g), of the water-absorbing agent with the weightratio of the water-absorbing agent in the absorbent matter.

[0258] This dynamic deterioration concentration absorption index is ascale as newly found by the present inventors as the index to predictthe absorption abilities of the absorbent matter in practical use.

[0259] If the weight ratio, γ, of the water-absorbing agent to the totalof the water-absorbing agent and the fibrous base material is selectedalong with the water-absorbing agent such that the dynamic deteriorationconcentration absorption index of equation (2) above can be 23 or more,then the absorption amount in a state near practical use of theresultant absorbent matter can be increased. Furthermore, ifwater-absorbing agents, of which the absorption capacity under no load,X (g/g), and the dynamic deterioration absorption capacity under a load,A (g/g), give the same dynamic deterioration concentration absorptionindex value as each other, are selected, then absorbent matters havingalmost the same absorption amount as each other in a state nearpractical use can be produced even if their absorption capacity valuesare different from each other. In addition, as is aforementioned, thedynamic deterioration absorption capacity under a load in this case is avalue as measured by a specific new evaluation process. As is mentionedabove, there are many prior art documents that disclose the evaluationof the absorption capacity under a load, in which the measurement isgenerally made in a comparatively short period of time using a liquidwith an electrolyte concentration near that of urine. However, in manycases, the actual wearing time of diapers extends for a long time of 6hours or more. Therefore, water-absorbent resins (water-absorbingagents), which provide excellent results with regard to the aboveconventional evaluation items as have been proposed so far, do notnecessarily exhibit excellent performance in practical use as well. Inaddition, urine contains compounds which change (deteriorate) theproperties of the resin with time, and the existence of these compoundsalso largely influences the absorption actions of the water-absorbentresin in practical use. In addition, because users move in practicaluse, dynamic force as well as load acts upon the resin. Furthermore, thepresent inventors have clarified that the degree of significance forsuch properties varies with the weight ratio, γ, of the water-absorbingagent to the total of the water-absorbing agent and the fibrous basematerial. That is to say, seeking after only the absorption capacityvalue under a load could not improve the absorption amount in a statenear practical use of absorbent matters in such as paper diaperscontaining fibrous base materials. For this improvement, it is necessaryto select the resin such that the dynamic deterioration concentrationabsorption index as defined in the present invention can satisfy thevalue of 23 or more.

[0260] As to the absorbent matter of the present invention, as theweight ratio γ of the water-absorbing agent to the total of thewater-absorbing agent and the fibrous base material gets smaller, theabsorption capacity under no load, X, tends to be more important forusable water-absorbing agents, but, considering the dynamicdeterioration concentration absorption index value, resins having a highvalue of dynamic deterioration absorption capacity under a load, A, canalso be used. In addition, as γ gets larger, the dynamic deteriorationabsorption capacity under a load, A, tends to be more important forusable water-absorbing agents, but, considering the dynamicdeterioration concentration absorption index value, resins having a highvalue of absorption capacity under no load, X, can be also used.Preferably, when γ is 0.4 or more, the effects of the present inventionare greatly exhibited. More preferably, γ is 0.6 or more. In the casewhere γ is less than 0.4, differences of the physical properties of somewater-absorbing agents are not greatly shown as differences of theperformances of absorbent matters.

[0261] In the present invention, the weight ratio, γ, of thewater-absorbing agent to the total of the water-absorbing agent and thefibrous base material is determined along with the water-absorbing agentsuch that the value of the dynamic deterioration concentrationabsorption index of equation (2) will be 23 or more. In the case wherethe dynamic deterioration concentration absorption index is less than23, the absorption amount in a state near practical use of the absorbentmatter is low: for example, in the case of paper diapers including theabsorbent matter, the probability of the occurrence of leakage is high.Preferably, the value of the dynamic deterioration concentrationabsorption index is 26 or more.

[0262] In addition, even if the value of the dynamic deteriorationconcentration absorption index is 23 or more, the amount of thewater-absorbing agent as used is preferably 8 (g) or more. An absorbentarticle, of which the amount of the water-absorbing agent as used issmaller than 8 (g), might lack the dry feeling as a product and displaya very large amount of desorption. The amount of the water-absorbingagent as used is more preferably in the range of 10˜20 (g). In addition,the basis weight of the water-absorbing agent in the absorbent matter ispreferably 100 (g/m²) or more.

[0263] The water-absorbing agent, as used for the absorbent matter ofthe present invention, preferably has an absorption speed of 20˜80 (sec)and a water-soluble content of 1˜15 weight % for the reason asaforementioned to explain the water-absorbing agent.

[0264] Incidentally, it is permissible to afford various functions tothe absorbent matter or article by further adding materials, such asdeodorants, perfumes, various inorganic powders, foaming agents,pigments, dyes, hydrophilic short fibers, fertilizers, oxidants,reductants, water, and salts, to the above-mentioned absorbent matter.

[0265] Absorbent Article

[0266] The above water-absorbing agent of the present invention isusable for the absorbent article. This absorbent article comprises anabsorbent layer, including the absorbent matter, and a surface sheetwith liquid permeability and a back sheet with liquid impermeability.The absorbent layer is interposed between the surface sheet with liquidpermeability and the back sheet with liquid impermeability. Because theabsorbent article comprises the absorbent layer including the absorbentmatter of the above-mentioned constitution, the absorbent article hasthe above-mentioned excellent water absorption properties. Specifiedexamples of the absorbent article include sanitary materials such aspaper diapers, sanitary napkins, and so-called incontinence pads, butthe absorbent article is not especially limited. Because the absorbentarticle has excellent water absorption properties, it can prevent urinefrom leaking and can afford so-called dry feeling in the case where theabsorbent article is, for example, a paper diaper. If necessary, it ispermissible that a diffusion layer, helping a liquid diffuse and, forexample, comprising nonwoven fabrics, cellulose, or crosslinkedcellulose, is put on the upper face of the absorbent layer or on theback or upper face of the surface sheet.

[0267] The constitution of the absorbent layer is not especially limitedif it has the above-mentioned absorbent matter. In addition, the processfor producing the absorbent layer is not especially limited.Furthermore, the method for interposing the absorbent layer between theliquid-permeable sheet and the liquid-impermeable sheet, namely, theprocess for producing the absorbent article, is not especially limited.

[0268] The absorbent matter, as included in the absorbent layer,comprises the above present invention water-absorbing agent and thefibrous base material. The explanation on the respective materialqualities, constitutions, weight ratios, and other properties of thewater-absorbing agent and the fibrous base material are omitted in thisportion of the specification because they are the same as thoseaforementioned to explain the absorbent matter.

[0269] The above-mentioned sheet with liquid permeability (hereinafterreferred to as liquid-permeable sheet) comprises a material that ispermeable with aqueous liquids. Examples of the material forming theliquid-permeable sheet include: nonwoven fabrics, woven fabrics; poroussynthetic resin films of polyethylene, polypropylene, polyester,polyamide. The above-mentioned sheet with liquid impermeability(hereinafter referred to as liquid-impermeable sheet) comprises amaterial that is impermeable with aqueous liquids. Examples of thematerial forming the liquid-impermeable sheet include: synthetic resinfilms of polyethylene, polypropylene, ethylene vinyl acetate, polyvinylchloride; films of combined materials of these synthetic resins withnonwoven fabrics; films of combined materials of the above-mentionedsynthetic resins with woven fabrics. Incidentally, theliquid-impermeable sheet may be permeable with steam.

[0270] Incidentally, it is permissible to afford various functions tothe absorbent matter or article by further adding materials, such asdeodorants, perfumes, various inorganic powders, foaming agents,pigments, dyes, hydrophilic short fibers, fertilizers, oxidants,reductants, water, and salts, to the above-mentioned absorbent matter.

[0271] The absorbent article of the present invention includes awater-absorbing agent having an absorption capacity of 30 (g/g) or moreunder no load and substantial absorption capacity (2) of 20 (g/g) ormore under a load, and further this absorbent article has a substantialconcentration absorption index of 23 or more, wherein when theabsorption capacity under no load of the water-absorbing agent isreferred to as X (g/g) and when substantial absorption capacity (2)under a load of the water-absorbing agent is referred to as Z (g/g) andwhen the weight ratio of the water-absorbing agent to the total of thewater-absorbing agent and the fibrous base material is referred to as P,the substantial concentration absorption index is shown by equation (2)below:

substantial concentration absorption index=X(1−β)+Zβ  (2).

[0272] The substantial concentration absorption index in the presentinvention is the sum of values as given by multiplying the absorptioncapacity under no load, X (g/g), and substantial absorption capacity (2)under a load, Z (g/g), of the water-absorbing agent by specific ratiosrespectively, and these specific ratios are determined from the weightratio, β, of the water-absorbing agent to the total of thewater-absorbing agent and the fibrous base material.

[0273] If the weight ratio, β, of the water-absorbing agent to the totalof the water-absorbing agent and the fibrous base material is selectedalong with the water-absorbing agent such that the substantialconcentration absorption index of equation (2) above can be 23 or more,then the absorption amount in a state near practical use of theresultant absorbent article can be increased. Furthermore, ifwater-absorbing agents, of which the absorption capacities under noload, X (g/g), and substantial absorption capacities (2) under a load, Z(g/g), give the same substantial concentration absorption index value aseach other, are selected, then absorbent articles having almost the sameabsorption amount as each other in a state near practical use can beproduced even if their absorption capacity values are different fromeach other. In addition, as is aforementioned, substantial absorptioncapacity (2) under a load in this case needs to be a value as measuredby a specific new evaluation process. As is mentioned above, there aremany prior art documents that disclose the evaluation of the absorptioncapacity under a load, in which the measurement is generally made in acomparatively short period of time using a liquid with an electrolyteconcentration near that of urine. However, in many cases, the actualwearing time of diapers extends for a long time of 6 hours or more.Therefore, water-absorbent resins (water-absorbing agents), whichprovide excellent results with regard to the above conventionalevaluation items as have been proposed so far, do not necessarilyexhibit excellent performance in practical use as well. Furthermore, thepresent inventors have clarified that the degree of significance forsuch properties varies with the weight ratio, β, of the water-absorbingagent to the total of the water-absorbing agent and the fibrous basematerial. That is to say, seeking after only the absorption capacityvalue under a load could not improve the absorption amount in a statenear practical use of absorbent articles such as paper diaperscontaining fibrous base materials. For this improvement, it is necessaryto select the resin such that the substantial concentration absorptionindex as defined in the present invention can satisfy the range of thepresent invention.

[0274] The absorbent article of the present invention comprises anabsorbent matter of which the weight ratio of the water-absorbing agentto the total of the water-absorbing agent and the fibrous base materialis β. When β is small, the absorption capacity under no load, X, tendsto be more important for usable water-absorbing agents, but, consideringthe substantial concentration absorption index value, resins having ahigh value of substantial absorption capacity (2) under a load, Z, canalso be used. In addition, when β is large, substantial absorptioncapacity (2) under a load, Z, tends to be more important for usablewater-absorbing agents, but, considering the substantial concentrationabsorption index value, resins having a high value of absorptioncapacity under no load, X, can be also used. Preferably, when β is 0.4or more, the effects of the present invention are greatly exhibited.More preferably, β is 0.6 or more. In the case where β is less than 0.4,differences of the physical properties of some water-absorbing agentsare not greatly shown as differences of the performances of absorbentarticles.

[0275] In the present invention, the weight ratio, β, of thewater-absorbing agent to the total of the water-absorbing agent and thefibrous base material is determined such that the value of thesubstantial concentration absorption index of equation (2) will be 23 ormore. In the case where the substantial concentration absorption indexis less than 23, the absorption amount in a state near practical use ofthe absorbent article is low, and when the absorbent article is, forexample, a paper diaper, the probability of the occurrence of leakage ishigh. Preferably, the value of the substantial concentration absorptionindex is 26 or more.

[0276] In addition, even if the value of the substantial concentrationabsorption index is 23 or more, the amount of the water-absorbing agentas used is preferably 8 (g) or more. An absorbent article, of which theamount of the water-absorbing agent as used is smaller than 8 (g), mightlack the dry feeling as a product and display a very large amount ofdesorption. The amount of the water-absorbing agent as used is morepreferably in the range of 10˜20 (g). In addition, the basis weight ofthe water-absorbing agent in the absorbent matter is preferably 100(g/m²) or more.

[0277] Such an absorbent article of the present invention can easily beproduced by using the present invention water-absorbing agent whichsatisfies the aforementioned parameters such as absorption capacityunder no load, deterioration absorption capacity under a load,deterioration shear absorption capacity under a load, and substantialabsorption capacity under a load.

[0278] In addition, as to the absorbent article of the presentinvention, an absorbent layer comprising the above-mentioned absorbentmatter is interposed between a liquid-permeable surface sheet and aliquid-impermeable back sheet, but it is permissible that a diffusionlayer, helping a liquid diffuse and, for example, comprising nonwovenfabrics, cellulose, or crosslinked cellulose, is put on the upper faceof the absorbent layer or on the back or upper face of the surfacesheet.

[0279] The absorbent article of the present invention comprises theabsorbent layer which includes the absorbent matter of theabove-mentioned constitution and is interposed between the sheet withliquid permeability and the sheet with liquid impermeability. Then,because the absorbent article comprises the absorbent layer includingthe absorbent matter of the above-mentioned constitution, the absorbentarticle has the above-mentioned excellent water absorption properties.Specified examples of the absorbent article include sanitary materialssuch as paper diapers, sanitary napkins, and so-called incontinencepads, but the absorbent article is not especially limited. Because theabsorbent article has excellent water absorption properties, it canprevent urine from leaking and can afford so-called dry feeling in thecase where the absorbent article is, for example, a paper diaper.

[0280] The above-mentioned sheet with liquid permeability (hereinafterreferred to as liquid-permeable sheet) comprises a material that ispermeable with aqueous liquids. Examples of the material forming theliquid-permeable sheet include: nonwoven fabrics, woven fabrics; poroussynthetic resin films of polyethylene, polypropylene, polyester,polyamide. The above-mentioned sheet with liquid impermeability(hereinafter referred to as liquid-impermeable sheet) comprises amaterial that is impermeable with aqueous liquids. Examples of thematerial forming the liquid-impermeable sheet include: synthetic resinfilms of polyethylene, polypropylene, ethylene vinyl acetate, polyvinylchloride; films of combined materials of these synthetic resins withnonwoven fabrics; films of combined materials of the above-mentionedsynthetic resins with woven fabrics. Incidentally, theliquid-impermeable sheet may be permeable with steam.

[0281] The constitution of the absorbent layer is not especially limitedif it has the above-mentioned absorbent matter. In addition, the processfor producing the absorbent layer is not especially limited.Furthermore, the method for interposing the absorbent layer between theliquid-permeable sheet and the liquid-impermeable sheet, namely, theprocess for producing the absorbent article, is not especially limited.

[0282] Incidentally, it is permissible to afford various functions tothe absorbent matter or article by further adding materials, such asdeodorants, perfumes, various inorganic powders, foaming agents,pigments, dyes, hydrophilic short fibers, fertilizers, oxidants,reductants, water, and salts, to the above-mentioned absorbent matter.

[0283] Absorption Property Measurement Process

[0284] The absorption property measurement process, according to thepresent invention, is a new evaluation process which is characterized inthat a liquid containing a reducible substance is used as a liquid to beabsorbed in a process for measuring at least one absorption propertyselected from the group consisting of: absorption properties under aload of a water-absorbing agent; absorption properties of an absorbentmatter of which the weight ratio of a water-absorbing agent to the totalof the water-absorbing agent and a fibrous base material is 0.4 or more;and absorption properties of an absorbent article including the aboveabsorbent matter.

[0285] Examples of the reducible substance as used above include:L-ascorbic acid; ascorbic acid salts such as sodium L-ascorbate;isoascorbic acid; isoascorbic acid salts; (bi)sulfurous acid salts suchas sodium sulfite and sodium hydrogensulfite; reducible metals (or saltsthereof) such as ferrous salts; and amines. L-ascorbic acid (or itssalts) and isoascorbic acid (or its salts) are preferable. Theconcentration of the liquid containing the reducible substance isdifferent according to the kind of the reducible substance as used oraccording to the aimed form of the use, but is usually in the range ofabout 0.001 to about 0.5 weight % when L-ascorbic acid is, for example,used as the reducible substance.

[0286] The liquid to be absorbed is not especially limited if itcontains the reducible substance, but examples thereof includeartificial urine, physiological sodium chloride solution, and humanurine.

[0287] As to conditions under which the absorption properties of thewater-absorbing agent, absorbent matter, and absorbent article aremeasured, it is preferable that they are measured at a temperature of,for example, 34˜42° C., more preferably 35˜39° C., and in the presenceof oxygen, for the purpose of predict absorption actions of theabsorbent article in practical use.

[0288] The absorption properties of the water-absorbing agent, asmeasured by the measurement process of the present invention, includeall absorption properties under a load, of which the examples includethe absorption capacity under a load and the liquid permeability under aload. The present invention is useful especially for the measurement ofthe absorption capacity under a load.

[0289] The conditions for measuring the absorption capacity under a loadmay, except the necessity of the step of absorbing the above liquidcontaining the reducible substance, be those where factors, such as loadconditions, weight of the resin, particle size of the resin, andpresence or absence of the liquid diffusion conditions, are optimizedconsidering the aimed form of the use in measurement processes forconventional absorption capacity under a load and diffusion absorptioncapacity under a load as are disclosed in documents such as EP 339,461,EP 605,150, EP 640,330, and EP 712,659. In a preferable embodiment, theresin is allowed to absorb the liquid containing the reducible substanceand to then stand stationary for a predetermined time, preferably 1˜12hours, and then the absorption capacity is measured under a load,because the absorption actions in practical use can thereby be judgedmore rightly.

[0290] Examples of the measurement of the liquid permeability under aload include the measurement of the permeability of the gel under a loadas disclosed in WO 95/26209.

[0291] The absorption properties of the absorbent matter, as measured bythe measurement process of the present invention, include all absorptionproperties under no load and under a load, of which the examples includethe absorption property (absorption amount) of the absorbent matterunder a load as disclosed in WO 95/26209, EP 339,461, and EP 712,659,and the absorption speed or Wet Back of the absorbent matter under aload as disclosed in EP 761,241.

[0292] The absorption properties of the absorbent article, as measuredby the measurement process of the present invention, include allabsorption properties under no load and under a load, of which theexamples include the absorption speed and amount of the absorbentarticle as disclosed in EP 339,461, and the absorption property(absorption amount) of the absorbent article as disclosed in EP 712,659.

[0293] By the measurement process of the present invention, thewater-absorbing agent, the absorbent matter, and the absorbent articlewhich exhibit always-stable absorption actions regardless of variationsof the liquid to be absorbed can be designed, selected, and picked out.In addition, the measurement process of the present invention can bepreferably used for the quality management on the production side of thewater-absorbent resin.

[0294] Effects and Advantages of the Invention

[0295] The water-absorbing agent, of which the absorption capacity underno load and the static or dynamic deterioration absorption capacityunder a load satisfy the respective values as specified in the presentinvention, has absorption properties that are stable to any compositionof urine and show little change with time. Therefore thiswater-absorbing agent is favorably used even for absorbent articleshaving high resin concentration.

[0296] The above water-absorbing agent is preferably obtained by addingthe ion blocking agent and/or the chelating agent to a water-absorbentresin in a specific way, or by adding the ion blocking agent and/or thechelating agent to a specific water-absorbent resin and mixing them, orby adding the chelating agent of a specific structure to awater-absorbent resin, so this water-absorbing agent undergoes littledeterioration due to urine with time and displays excellent absorptionproperties.

[0297] The absorbent article of the present invention is specified bythe static or dynamic deterioration concentration absorption indexconsidering the resin concentration, so this absorbent article displaysan always stable high absorption amount, especially, a high absorptionamount till the leakage occurs in a used state very near to practicaluse.

[0298] The absorption property measurement process of the presentinvention enables easy and precise prediction of the absorption actionsof the water-absorbing agent or absorbent article in practical use, sothis process is very useful for producing a water-absorbing agent orabsorbent article that displays excellent absorption properties.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0299] Hereinafter, the present invention is more specificallyillustrated by the following examples of some preferred embodiments incomparison with comparative examples not according to the invention.However, the invention is not limited to these examples. Incidentally,the performances of the water-absorbing agent were measured by thefollowing methods:

[0300] (a) Absorption Capacity Under no Load

[0301] First, 0.2 g of water-absorbing agent (water-absorbent resin) wasuniformly placed into a nonwoven-fabric-made bag (60 mm×60 mm) and thenimmersed into a 0.9 wt % aqueous sodium chloride solution (physiologicalsodium chloride solution). Sixty minutes later, the bag was drawn up andthen drained at 250 G for 3 minutes with a centrifuge, and the weight W₁(g) of the bag was then measured. On the other hand, the same procedurewas carried out using no water-absorbing agent, and the resultant weightW₀ (g) was measured. Thus, the absorption capacity (g/g) under no loadwas calculated from these weights W₁ and W₀ in accordance with thefollowing equation:

absorption capacity (g/g)={(weight W ₁ (g)−weight W ₀ (g))/(weight (g)of water-absorbing agent)}−1.

[0302] (b) Absorption Capacity Under Load

[0303] The absorption capacity under a load was measured under a load of50 g/cm² in accordance with ABSORBENCY AGAINST PRESSURE, ABSORBENCY III442.1˜99 (October 1997) of EDANA. That is to say, 0.9 g ofwater-absorbing agent (water-absorbent resin) is uniformly spread on astainless wire net of 400 mesh (mesh size: 38 μm) as attached by fusionto the bottom of a plastic supporting cylinder of inner diameter 60 mm.Then, a piston (cover plate) (which has an outer diameter only a littlesmaller than 60 mm and makes no gap with the wall face of the supportingcylinder, but is not hindered from moving up and down) is mounted on thewater-absorbing agent, and the total weight (Wa (g)) of the supportingcylinder, the water-absorbing agent and the piston is measured. Next, aload, as adjusted to uniformly apply a load of 50 g/cm² (including theweight of the piston) to the water-absorbing agent, is mounted on thepiston, thus completing a set of measurement apparatus. A glass filterof 90 mm in diameter is mounted inside a Petri dish of 150 mm indiameter, and a 0.9 wt % aqueous sodium chloride solution is added up tothe same level as the upper face of the glass filter, on which a filterpaper of diameter 9 cm (No. 2 made by Toyo Filter Paper Co., Ltd.) isthen mounted such that its entire surface will be wetted, and theexcessive liquid is removed.

[0304] The above set of measurement apparatus is mounted on the abovewet filter paper, thereby allowing the water-absorbing agent to absorbthe liquid under a load. After the liquid face has fallen under theupper part of the glass filter, the liquid is added to keep the liquidface level constant. After 1 hour, the set of measurement apparatus isremoved by lifting it, and the weight (Wb (g)) (the total weight of thesupporting cylinder, the swollen water-absorbing agent and the piston)as freed from the load is measured again. Thus, the absorption capacity(g/g) under a load was calculated from the above weights Wa and Wb inaccordance with the following equation:

Absorption capacity under load (g/g)=(Wb (g)−Wa (g))/(weight ofwater-absorbing agent)(g).

[0305] (c) Static Deterioration Absorption Capacity (1) Under Load

[0306] Static deterioration absorption capacity (1) under a load wasmeasured using the same measurement apparatus as described in the aboveitem of the absorption capacity under load. The measurement process isdescribed below. First of all, 0.9 g of water-absorbing agent(water-absorbent resin) is uniformly spread inside the supportingcylinder, namely, on the stainless wire net of 400 mesh, and the weightas given by adding thereto the above piston (cover plate) is referred toas W1 (g). Then, 13.5 g of a 0.9 wt % aqueous sodium chloride solution,containing L-ascorbic acid in a concentration of 0.005 wt %, is addedinto a Petri dish of 90 mm in diameter as prepared separately, on whichthe supporting cylinder, with the above water-absorbent resin spread onthe wire net of the bottom and provided with no load, is then mounted,thus allowing the resin to uniformly absorb the 0.9 wt % aqueous sodiumchloride solution containing L-ascorbic acid in a concentration of 0.005wt % in the Petri dish and to thereby form a gel as swollen to 15 timesand to then stand stationary at 37° C. for 6 hours.

[0307] After 6 hours, the above piston (cover plate) and the load, asadjusted to uniformly apply a load of 50 g/cm² to the above swollenwater-absorbing agent, are mounted on the swollen water-absorbing agentin this order. Next, a glass filter of 90 mm in diameter is mountedinside a Petri dish of 150 mm in diameter, and a 0.9 wt % aqueous sodiumchloride solution is added up to the same level as the surface of theglass filter, on which a filter paper of diameter 9 cm (No. 2 made byToyo Filter Paper Co., Ltd.) is then mounted such that its entire upperface will be wetted, and the excessive liquid is removed.

[0308] Then, the above set of measurement apparatus, applying a pressureto the gel as swollen to 15 times, is mounted on the above wet filterpaper, thereby allowing the gel to absorb the liquid under a load. Afterthe liquid face has fallen under the upper part of the glass filter, theliquid is added to keep the liquid face level constant. After 1 hour,the set of measurement apparatus is lifted to thereby be removed fromthe filter paper, and is then released from the load to measure theresultant weight (W2 (g)) again. Then, static deterioration absorptioncapacity (1) under a load was calculated from the above weights W1 andW2 in accordance with the following equation:

Static deterioration absorption capacity (1) under load (g/g)=(W2 (g)−W1(g))/(weight of water-absorbing agent)(g).

[0309] (d) Static Deterioration Absorption Capacity (2) Under Load

[0310] Static deterioration absorption capacity (2) under a load wascalculated in the same way as of the above measurement of staticdeterioration absorption capacity (1) under a load, except that theduration of 6 hours for which the gel, as swollen to 15 times, wasallowed to stand stationary was changed to 2 hours.

[0311] (e) Static Deterioration Absorption Capacity (3) Under Load

[0312] Static deterioration absorption capacity (3) under a load wascalculated in the same way as of the above measurement of staticdeterioration absorption capacity (2) under a load, except that theconcentration, 0.005 wt %, of L-ascorbic acid in the physiologicalsodium chloride solution was changed to 0.05 wt %.

[0313] (f) Static Deterioration Absorption Capacity (4) Under Load

[0314] Static deterioration absorption capacity (4) under a load wascalculated in the same way as of the above measurement of staticdeterioration absorption capacity (1) under a load, except that theconcentration, 0.005 wt %, of L-ascorbic acid in the physiologicalsodium chloride solution was changed to 0.05 wt %, and that the load of50 g/cm² to uniformly apply to the water-absorbing agent was changed to20 g/cm².

[0315] (g) Dynamic Deterioration Absorption Capacity Under Load

[0316] First, 0.9 g of water-absorbing agent (water-absorbent resin) wasplaced into a polyethylene bag of 5 cm×10 cm, and then 13.5 g of asolution as prepared by dissolving L-ascorbic acid into a 0.9 wt %aqueous sodium chloride solution (wt % is based on the weight of thesolution) in a concentration of 0.005 wt % was added into the bag, thuspreparing a gel as swollen to 15 times, and then the bag was sealed. Thetemperature of the resultant sealed product was kept at 37° C. for 4hours. Thereafter, air was extracted from the bag. The bag was sealedagain, and dynamic damage was done to the gel with a 5-kg-heavy roller(diameter 9 cm, width 20 cm) along with the bag, when the roller was runback and forth 50 times each at 5 seconds/revolution.

[0317] The gel, to which dynamic damage had been done in the above way,was got out of the bag, and the absorption capacity of this gel under aload was measured with the same measurement apparatus as stated in theabove item (b) of absorption capacity under load, and the resultantmeasurement value was regarded as the dynamic deterioration absorptioncapacity under a load. The measurement process is as follows. Thedynamically damaged gel, as had been got out of the bag above, wasuniformly spread inside the supporting cylinder, namely, on thestainless wire net of 400 mesh, and the weight (W_(A) (g)) as given byadding thereto the above piston (cover plate) was measured. Then, aload, as adjusted to uniformly apply a load of 50 g/cm² (including theweight of the above piston) to the gel, was mounted on the piston. Next,a glass filter of 90 mm in diameter was mounted inside a Petri dish of150 mm in diameter, and a 0.9 wt % aqueous sodium chloride solution wasadded up to the same level as the surface of the glass filter, on whicha filter paper of diameter 9 cm (No. 2 made by Toyo Filter Paper Co.,Ltd.) was then mounted such that its entire upper face of the filterpaper would be wetted, and the excessive liquid was removed.

[0318] Then, the above set of measurement apparatus, applying a pressureto the gel as swollen to 15 times, was mounted on the above wet filterpaper, thereby allowing the gel to absorb the liquid under a load. Afterthe liquid face had fallen under the upper part of the glass filter, theliquid was added to keep the liquid face level constant. After 1 hour,the set of measurement apparatus was lifted to thereby be removed fromthe filter paper, and was then released from the load to measure theresultant weight (W_(B) (g)) again. Then, the dynamic deteriorationabsorption capacity under a load was calculated from the above weightsW_(A) and W_(B) in accordance with the following equation:

dynamic deterioration absorption capacity under load (g/g)=(W _(B) (g)−W_(A) (g)+13.5)g/(weight of water-absorbing agent)g.

[0319] (h) Dynamic Absorption Capacity Under Load

[0320] The dynamic absorption capacity under a load was calculated fromthe above weights WA and WB in accordance with the below-mentionedequation in the same way as of the above measurement (g) of the dynamicdeterioration absorption capacity under a load, except that a 0.9 wt %aqueous sodium chloride solution (wt % is based on the weight of thesolution) free of L-ascorbic acid was used as the solution to swell thewater-absorbing agent before doing dynamical damage, and that the sealedproduct of the gel as swollen to 15 times was kept at 37° C. for 30minutes.

Dynamic absorption capacity under load (g/g)=(W _(B) (g)−W _(A)(g)+13.5)g/(weight of water-absorbing agent)g

[0321] (i) Substantial Absorption Capacity (1) Under Load

[0322] Substantial absorption capacity (1) under a load was measuredusing the same measurement apparatus as described in the above item ofthe absorption capacity under a load. The measurement process isdescribed below. First of all, 0.9 g of water-absorbing agent(water-absorbent resin) is uniformly spread inside the supportingcylinder, namely, on the stainless wire net of 400 mesh, and the weightas given by adding thereto the above piston (cover plate) is referred toas W1 (g). Then, 13.5 g of a 0.9 wt % aqueous sodium chloride solutionis added into a Petri dish of 90 mm in diameter as prepared separately,on which the supporting cylinder, with the above water-absorbent resinspread on the wire net of the bottom and provided with no load, is thenmounted, thus allowing the resin to uniformly absorb the 0.9 wt %aqueous sodium chloride solution in the Petri dish and to thereby form agel as swollen to 15 times and to then stand stationary at 37° C. for 2hours.

[0323] After 2 hours, the above piston (cover plate) and the load, asadjusted to uniformly apply a load of 50 g/cm² to the above swollenwater-absorbing agent, are mounted on the swollen water-absorbing agentin this order. Next, a glass filter of 90 mm in diameter is mountedinside a Petri dish of 150 mm in diameter, and a 0.9 wt % aqueous sodiumchloride solution is added up to the same level as the surface of theglass filter, on which a filter paper of diameter 9 cm (No. 2 made byToyo Filter Paper Co., Ltd.) is then mounted such that its entire upperface will be wetted, and the excessive liquid is removed.

[0324] Then, the above set of measurement apparatus, applying a pressureto the gel as swollen to 15 times, is mounted on the above wet filterpaper, thereby allowing the gel to absorb the liquid under a load. Afterthe liquid face has fallen under the upper part of the glass filter, theliquid is added to keep the liquid face level constant. After 1 hour,the set of measurement apparatus is lifted to thereby be removed fromthe filter paper, and is then released from the load to measure theresultant weight (W2 (g)) again. Then, substantial absorption capacity(1) under a load was calculated from the above weights WI and W2 inaccordance with the following equation:

substantial absorption capacity (1) under load (g/g)=(W2 (g)−W1(g))/(weight of water-absorbing agent)(g).

[0325] (j) Substantial Absorption Capacity (2) Under Load

[0326] Substantial absorption capacity (2) under a load was calculatedin the same way as of the above measurement of substantial absorptioncapacity (1) under a load, except that the duration of 2 hours for whichthe gel, as swollen to 15 times, was allowed to stand stationary waschanged to 6 hours.

[0327] (k) Absorption Speed

[0328] The measurement of the absorption speed was carried out inaccordance with JIS K7224. Next, the measurement process is described.First, 50.0 g of physiological sodium chloride solution (0.9 wt %aqueous sodium chloride solution), as had been adjusted to 30° C., and astirring chip (which had a central diameter of 8 mm, a diameter of 7 mm,and a length of 30 mm and had been coated with fluororesin) were placedinto a beaker of 100 ml with a flat bottom as regulated by JIS R3503,and then stirred at a rate of 600 rpm with a magnetic stirrer. Then, 2 gof water-absorbing agent was added into the beaker, so that gelation wascaused by water absorption swelling action, and when the fluiditydecreased and finally the water vortex of the stirring centerdisappeared, namely, when the stirring chip became invisible, wasregarded as the end point. The time, as spent since the addition of thesample till the disappearance of the vortex, was measured and regardedas the absorption speed.

[0329] (l) Water-soluble Content

[0330] First, 0.500 g of water-absorbent resin was dispersed into 1,000ml of deionized water and stirred for 16 hours, and then filtered withfilter paper. Next, 50 g of the resultant filtrate was placed into a 100ml beaker, and 1 ml of a 0.1 N aqueous sodium hydroxide solution, 10.00ml of an N/200 aqueous methyl glycol chitosan solution, and 4 drops of a0.1 wt % aqueous Toluidine Blue solution were added to the filtrate.Next, the resultant solution in the beaker was subjected to colloidtitration with an N/400 aqueous potassium polyvinylsulfate solution todetermine titration amount Y (ml) assuming that the moment at which thecolor of the solution changed from blue to red purple was the terminalof the titration. In addition, titration amount Z (ml) was determined bycarrying out blank titration in the same way as the above-mentioned,except that 50 g of the filtrate was replaced with 50 g of deionizedwater. Then, the water-soluble content (wt %) was calculated fromtitration amounts Y and Z and from neutralization ratio W (mol %) of theacrylic acid, as provided for the production of the water-absorbentresin, in accordance with the following equation:

water-soluble content (wt %)=(Z (ml)−Y (ml))×0.01×72×(100−W (mol%))+(94W (mol %)/100).

[0331] (m) Water Content (On the Wet Basis)

[0332] About 1 g of water-absorbent resin was heated in an oven of 105°C. for 3 hours, and the weight W (g) of the water-absorbent resin weremeasured before and after heating, and the water content (wt %) (on thewet basis) was calculated in accordance with the following equation:

water content (wt %)=(W _(before) (g)−W _(after) (g))/W _(before) (g)

[0333] wherein:

[0334] W_(before) is the weight of the water-absorbent resin beforedrying; and W_(after) is the weight of the water-absorbent resin afterdrying.

EXAMPLE 1

[0335] A reaction solution was prepared by dissolving 9.25 g ofpolyethylene glycol diacrylate (average molar number of added ethyleneoxide: 8) into 5,500 g of an aqueous solution of sodium acrylate with aneutralization ratio of 65 mol % (monomer concentration: 30 wt %). Next,this solution was degassed under a nitrogen gas atmosphere for 30minutes, and then supplied into a reaction vessel as prepared by cappinga stainless-steel-made double-arm type kneader of a capacity of 10liters having two sigma type vanes and a jacket. While maintaining thereaction solution at 30° C., the atmosphere inside the system wasreplaced with a nitrogen gas. Next, while the reaction solution wasstirred, 1.91 g of 2,2′-azobis(2-amidinopropane) dihydrochloride, 0.96 gof sodium persulfate, and 0.10 g of L-ascorbic acid were added, so thata polymerization reaction got started about 1 minute after. Thepolymerization was carried out at 30-80° C., and the resultant hydrogelpolymer was got out 60 minutes after the initiation of thepolymerization.

[0336] The resultant hydrogel polymer had a finely divided diameter ofabout 5 mm. This finely divided hydrogel polymer was spread on a 50-meshwire net and dried at 150° C. with hot air for 90 minutes. Then, theresultant dried product was pulverized with a vibration mill and furtherclassified with a wire net of 20 mesh, thus obtaining water-absorbentresin precursor (a) as pulverized into the irregular shape with anaverage particle diameter of 300 μm.

[0337] A surface-crosslinking agent, comprising 0.005 weight parts ofpentasodium diethylenetriaminepentaacetate, 1 weight part of propyleneglycol, 0.05 weight parts of ethylene glycol diglycidyl ether, 3 weightparts of water, and 1 weight part of isopropyl alcohol, was mixed with100 weight parts of water-absorbent resin precursor (a) as obtainedabove. The resultant mixture was heated at 210° C. for 45 minutes, thusobtaining water-absorbing agent (1), for which the measurement resultsof the following properties are shown in Table 1: absorption capacityunder no load, absorption capacity under load, static deteriorationabsorption capacities (1)˜(4) under load, dynamic absorption capacityunder load, dynamic deterioration absorption capacity under load,substantial absorption capacities (1)˜(2) under load, absorption speed,and water-soluble content.

EXAMPLE 2

[0338] A reaction solution was prepared by charging 720 g of acrylicacid, 3.08 g of N,N′-methylenebisacrylamide as the internal-crosslinkingagent, and 2,718 g of deionized water as the solvent into a reactionvessel as prepared by capping a stainless-steel-made double-arm typekneader of a capacity of 10 liters having two sigma type vanes and ajacket. Next, while maintaining the temperature of this reactionsolution at 15° C., the atmosphere inside the system was replaced with anitrogen gas. Then, while the reaction solution was stirred, 21.6 g of a10 wt % aqueous 2,2′-azobis(2-amidinopropane) dihydrochloride solution,18.0 g of a 1 wt % aqueous L-ascorbic acid solution, 20.6 g of a 3.5-%aqueous hydrogen peroxide solution were added to initiate apolymerization reaction. The stirring was stopped at the same time asthe initiation of the polymerization reaction. Then, the polymerizationreaction was carried out while the temperature of the jacket was fitlyelevated with the temperature rising of the reaction solution such thatthe temperatures of the reaction solution and the jacket would be almostthe same as each other. Then, after the temperature of the reactionsolution had reached its peak temperature, the temperature of thereaction solution was maintained at not lower than 55° C. by controllingthe temperature of the jacket. After 3 hours, the resultant hydrogelcrosslinking polymer was pulverized by rotating the vanes of thedouble-arm type kneader. Furthermore, the temperature was kept at about50° C. while rotating the vanes of the double-arm type kneader, and 750g of a 40 wt % aqueous sodium hydroxide solution was dropped and mixed,thus obtaining a hydrogel polymer with a neutralization ratio of 75 mol%, when the time as needed for the neutralization was 6 hours. Thishydrogel polymer was spread on a wire net of 50 mesh and then dried witha hot air of 60° C. for 16 hours. Next, the resultant dry product waspulverized with a vibration mill and further classified with a wire netof 20 mesh, thus obtaining water-absorbent resin precursor (b) aspulverized into the irregular shape with an average particle diameter of300 μm.

[0339] A surface-crosslinking agent, comprising 1 weight part ofpropylene glycol, 0.05 weight parts of ethylene glycol diglycidyl ether,3 weight parts of water, and 1 weight part of isopropyl alcohol, wasmixed with 100 weight parts of water-absorbent resin precursor (b) asobtained above. The resultant mixture was heated at 205° C. for 50minutes, thus obtaining water-absorbent resin b, of which the absorptioncapacity under load was 26.9 (g/g) and the water content (on the wetbasis) was 1 wt % or less. Then, 100 weight parts of thiswater-absorbent resin b was sprayed with a mixed solution, comprising0.005 weight parts of pentasodium diethylenetriaminepentaacetate and 3weight parts of water, and then dried at 80° C., thus obtainingwater-absorbing agent (2) according to the present invention, for whichthe measurement results of the following properties are shown in Table1: absorption capacity under no load, absorption capacity under load,static deterioration absorption capacities (1)˜(4) under load, dynamicabsorption capacity under load, dynamic deterioration absorptioncapacity under load, substantial absorption capacities (1)˜(2) underload, absorption speed, and water-soluble content.

Comparative Example 1

[0340] A reaction solution was prepared by dissolving 1.52 weight partsof trimethylolpropane triacrylate into 5,500 weight parts of an aqueoussolution of sodium acrylate with a neutralization ratio of 75 mol %(monomer concentration: 33 wt %). Next, this solution was degassed undera nitrogen gas atmosphere for 30 minutes, and then supplied into areaction vessel as prepared by capping a stainless-steel-made double-armtype kneader of a capacity of 10 liters having two sigma type vanes anda jacket. While maintaining the reaction solution at 30° C., theatmosphere inside the system was replaced with a nitrogen gas. Next,while the reaction solution was stirred, 2.46 weight parts of sodiumpersulfate and 0.10 weight parts of L-ascorbic acid were added, so thata polymerization reaction got started about 1 minute after. Thepolymerization was carried out at 30˜80° C., and the resultant hydrogelpolymer was got out 60 minutes after the initiation of thepolymerization. The resultant hydrogel polymer had a finely divideddiameter of about 5 mm. This finely divided hydrogel polymer was spreadon a 50-mesh wire net and dried at 150° C. with hot air for 90 minutes.Then, the resultant dried product was pulverized with a vibration milland further classified with a wire net of 20 mesh, thus obtainingwater-absorbent resin precursor (c) as pulverized into the irregularshape with an average particle diameter of 350 μm.

[0341] A surface-crosslinking agent, comprising 1 weight part ofglycerol, 0.05 weight parts of ethylene glycol diglycidyl ether, 3weight parts of water, and 1 weight part of isopropyl alcohol, was mixedwith 100 weight parts of water-absorbent resin precursor (c) as obtainedabove. The resultant mixture was heated at 195° C. for 40 minutes, thusobtaining water-absorbent resin c, of which the absorption capacityunder load was 22.3 (g/g) and the water content (on the wet basis) was 1wt % or less. Then, 100 weight parts of this water-absorbent resin c wassprayed with a mixed solution, comprising 0.005 weight parts ofpentasodium diethylenetriaminepentaacetate and 3 weight parts of water,and then dried at 80° C., thus obtaining comparative water-absorbingagent (1), for which the measurement results of the following propertiesare shown in Table 1: absorption capacity under no load, absorptioncapacity under load, static deterioration absorption capacities (1)˜(4)under load, dynamic absorption capacity under load, dynamicdeterioration absorption capacity under load, substantial absorptioncapacities (1)˜(2) under load, absorption speed, and water-solublecontent.

Comparative Example 2

[0342] A reaction solution was prepared by dissolving 4.5 weight partsof polyethylene glycol diacrylate (average molar number of addedethylene oxide: 8) into 5,500 weight parts of an aqueous solution ofsodium acrylate with a neutralization ratio of 75 mol % (monomerconcentration: 33 wt %). Next, this solution was degassed under anitrogen gas atmosphere for 30 minutes, and then supplied into areaction vessel as prepared by capping a stainless-steel-made double-armtype kneader of a capacity of 10 liters having two sigma type vanes anda jacket. While maintaining the reaction solution at 30° C., theatmosphere inside the system was replaced with a nitrogen gas. Next,while the reaction solution was stirred, 2.46 weight parts of sodiumpersulfate and 0.10 weight parts of L-ascorbic acid were added, so thata polymerization reaction got started about 1 minute after. Thepolymerization was carried out at 30˜80° C., and the resultant hydrogelpolymer was got out 60 minutes after the initiation of thepolymerization. The resultant hydrogel polymer had a finely divideddiameter of about 5 mm. This finely divided hydrogel polymer was spreadon a 50-mesh wire net and dried at 150° C. with hot air for 90 minutes.Then, the resultant dried product was pulverized with a vibration milland further classified with a wire net of 20 mesh, thus obtainingwater-absorbent resin precursor (d) as pulverized into the irregularshape with an average particle diameter of 280 μm. Asurface-crosslinking agent, comprising 1 weight part of propyleneglycol, 0.05 weight parts of ethylene glycol diglycidyl ether, 3 weightparts of water, and 1 weight part of isopropyl alcohol, was mixed with100 weight parts of water-absorbent resin precursor (d) as obtainedabove. The resultant mixture was heated at 210° C. for 40 minutes, thusobtaining comparative water-absorbing agent (2), for which themeasurement results of the following properties are shown in Table 1:absorption capacity under no load, absorption capacity under load,static deterioration absorption capacities (1)˜(4) under load, dynamicabsorption capacity under load, dynamic deterioration absorptioncapacity under load, substantial absorption capacities (1)˜(2) underload, absorption speed, and water-soluble content.

EXAMPLE 3

[0343] An aqueous monomer solution was prepared by mixing 67.0 weightparts of a 37 wt % aqueous sodium acrylate solution, 10.2 weight partsof acrylic acid, 0.097 weight parts of polyethylene glycol diacrylate(average number of polyethylene oxide units: 8), and 22.0 weight partsof water together. Nitrogen was blown into the above aqueous monomersolution in a vat, thus reducing the concentration of dissolved oxygenin the solution to 0.1 ppm or below. Then, the temperature of thesolution was adjusted to 18° C. under nitrogen atmosphere. Next, 0.16weight parts of a 5 wt % aqueous sodium persulfate solution, 0.16 weightparts of a 5 wt % aqueous 2,2′-azobis (2-amidinopropane) hydrochloridesolution, 0.15 weight parts of a 0.5 wt % aqueous L-ascorbic acidsolution, and 0.17 weight parts of a 0.35 wt % aqueous hydrogen peroxidesolution were dropped in sequence under stirring.

[0344] Immediately after the dropping of hydrogen peroxide, apolymerization reaction got started, and after another 10 minutes, thetemperature of the monomer reached the peak temperature. The peaktemperature was 85° C. Then, the vat was immersed into a hot water bathof 80° C. and aged for 15 minutes.

[0345] The resultant transparent hydrogel was crushed with a meatchopper, and the resultant finely divided hydrogel polymer was spread ona 50-mesh wire net and dried at 160° C. with hot air for 65 minutes.Then, the resultant dry product was pulverized with a pulverizingmachine and then classified into what passed through a screen of 850 μm,but remained on a screen of 106 Em, thus obtaining water-absorbent resinprecursor (e) as pulverized into the irregular shape with an averageparticle diameter of 320 μm.

[0346] A surface-crosslinking agent, comprising 1 weight part ofpropylene glycol, 0.5 weight parts of 1,4-butanediol, 3 weight parts ofwater, and 1 weight part of isopropyl alcohol, was mixed with 100 weightparts of water-absorbent resin precursor (e) as obtained above. Theresultant mixture was heated at 210° C. for 40 minutes, thus obtainingwater-absorbent resin e, of which the absorption capacity under load was26.6 (g/g) and the water content (on the wet basis) was 1 wt % or less.Then, 100 weight parts of this water-absorbent resin e was sprayed witha mixed solution, comprising 0.005 weight parts of pentasodiumdiethylenetriaminepentaacetate and 3 weight parts of water, and thendried at 80° C., thus obtaining water-absorbing agent (3) according tothe present invention, for which the measurement results of thefollowing properties are shown in Table 1: absorption capacity under noload, absorption capacity under load, static deterioration absorptioncapacities (1)˜(4) under load, dynamic absorption capacity under load,dynamic deterioration absorption capacity under load, substantialabsorption capacities (1)˜(2) under load, absorption speed, andwater-soluble content.

Comparative Example 3

[0347] A reaction solution was prepared by dissolving 10.6 g ofpolyethylene glycol diacrylate into 6,570 g of a 30 wt % aqueoussolution of partially neutralized sodium acrylate with a neutralizationratio of 75 mol %. Next, this reaction solution was provided to areactor having a structure such that a cover was equipped to a stainlesstwin-arm type kneader of 10 L in capacity having two sigma type vanesand a jacket. The internal atmosphere of the reactor was replaced withnitrogen while the temperature of the reaction solution was kept at 30°C. by circulating water of 30° C. in the jacket. Next, 15.6 g of a 20 wt% aqueous sodium persulfate solution and 14.9 g of a 0.1 wt % aqueousL-ascorbic acid solution were added as polymerization initiators intothe reactor while a blade of the kneader was stirred at 40 rpm, thusinitiating polymerization. When the initiation of the polymerization wasconfirmed from clouding of the reaction mixture, the blade was stopped,and the reaction mixture was then left as it was until the internaltemperature fell to 60° C. due to the removal of the heat using thejacket. When the internal temperature further fell below 60° C., theblade was rotated to disintegrate the resultant gel, and then thepolymerization was further carried out such that the peak of theinternal temperature would be 75° C. Then, the jacket temperature wasraised to 60° C., and while the gel was integrated, the polymerizationsystem was kept at 65° C. or higher for 20 minutes, thereby completingthe polymerization.

[0348] The resultant hydrogel polymer was dried at 160° C. with hot airfor 65 minutes. Then, the resultant dry product was pulverized with avibration mill, thus obtaining water-absorbent resin precursor (f) aspulverized into the irregular shape with an average particle diameter of450 μm.

[0349] A surface-crosslinking agent, comprising 0.5 weight parts ofglycerol, 0.05 weight parts of ethylene glycol diglycidyl ether, 3weight parts of water, and 0.75 weight parts of isopropyl alcohol, wasmixed with 100 weight parts of water-absorbent resin precursor (f) asobtained above. The resultant mixture was heated at 200° C. for 50minutes, thus obtaining comparative water-absorbing agent (3), for whichthe measurement results of the following properties are shown in Table1: absorption capacity under no load, absorption capacity under load,static deterioration absorption capacities (1)˜(4) under load, dynamicabsorption capacity under load, dynamic deterioration absorptioncapacity under load, substantial absorption capacities (1)˜(2) underload, absorption speed, and water-soluble content. TABLE 1 Com- Com-Com- parative parative parative Example 1 Example 2 Example 3 ExampleCom- Com- Com- Example 1 2 parative parative Example 3 parative Water-Water- water- water- Water- water- absorbing absorbing absorbingabsorbing absorbing absorbing Water-absorbing agent used agent 1 agent 2agent 1 agent 2 agent 3 agent 3 Absorption capacity under 31.6 37.5 35.531.3 34.4 35.5 no load (g/g) Absorption capacity under 26.8 26.3 22.626.7 26.4 27.6 load (g/g) Absorption speed (sec) 42 35 45 31 43 83Water-soluble content (%) 6 2 25 12 5 6 Substantial absorption 24.8 25.016.7 21.6 25.1 — capacity (1) under load (g/g) Substantial absorption24.8 23.2 15.7 19.3 24.3 — capacity (2) under load (g/g) Staticdeterioration 24.7 23.5 15.4 18.8 24.1 19.2 absorption capacity (1)under load (g/g) Static deterioration 25.9 25.2 16.8 22.0 24.9 22.5absorption capacity (2) under load (g/g) Static deterioration 24.5 24.815.9 18.5 24.2 19.0 absorption capacity (3) under load (g/g) Staticdeterioration 32.1 36.2 24.8 16.4 34.2 26.0 absorption capacity (4)under load (g/g) Dynamic absorption capacity 25.3 26.0 16.5 25.3 25.526.1 under load (g/g) Dynamic deterioration 22.3 23.4 15.4 15.6 22.519.2 absorption capacity under load (g/g)

EXAMPLE 4

[0350] First, 50 weight parts of water-absorbing agent (1), as obtainedin Example 1, and 50 weight parts of wood-pulverized pulp were mixedtogether in a dry manner with a mixer. Next, the resultant mixture wasshaped into a web of the size of 120 mm×380 mm by pneumatically moldingthe mixture on a wire screen of 400 mesh (mesh size: 38 μm) with a batchtype pneumatic device. In addition, this web was pressed for 5 secondsunder a pressure of 2 kg/cm², thus obtaining an absorbent matter of aweight of about 526 g/m².

[0351] Next, a back sheet (liquid-impermeable sheet) of aliquid-impermeable polypropylene with a so-called leg gather, theabove-mentioned absorbent matter, and a top sheet (liquid-permeablesheet) of a liquid-permeable polypropylene were attached to each otherin this order with double coated tapes, and two so-called tape fastenerswere then provided to the resultant attached product, thus obtaining anabsorbent article (i.e. paper diaper). The weight of this absorbentarticle was 47 g.

[0352] This absorbent article was fitted up to each of four units ofso-called kewpie dolls (three units of which had a body length of 55 cmand a weight of 5 kg, and the other one unit had a body length of 65 cmand a weight of 6 kg), and these dolls were laid on their faces at roomtemperature of 37° C. Then, a tube was inserted between the absorbentarticle and the dolls, and 50 g of a physiological sodium chloridesolution containing L-ascorbic acid in a concentration of 0.005 wt % wasinjected through the tube every 90 minutes to a position correspondingto where urine is discharged from the human body. Then, this injectionoperation was ended when the injected physiological sodium chloridesolution began leaking without being absorbed by the absorbent article,and the amount of the physiological sodium chloride solution, as hadbeen injected until then, was measured, and the average value thereoffor the above-mentioned four units of kewpie dolls was regarded as theabsorption amount of the absorbent article in a state of lying facedown. The result of the absorption amount (g) in a state of lying facedown is shown in Table 2 along with values of deterioration absorptionindex under load, substantial concentration absorption index, and staticand dynamic deterioration concentration absorption indices.

EXAMPLES 5 and 6

[0353] Absorbent articles were obtained in the same way as of Example 4except that water-absorbing agent (1) was replaced with water-absorbingagents (2) and (3) as obtained in Examples 2 and 3 respectively. Boththe resultant absorbent articles weighed 47 g.

[0354] The absorption amount of each of these absorbent articles in astate of lying face down was determined in the same way as of Example 4.The result of the absorption amount (g) in a state of lying face down isshown in Table 2 along with values of deterioration absorption indexunder load, substantial concentration absorption index, and static anddynamic deterioration concentration absorption indices.

Comparative Examples 4, 5, and 6

[0355] Comparative absorbent articles were obtained in the same way asof Example 4 except that water-absorbing agent (1) was replaced withcomparative water-absorbing agents (1), (2), and (3) as obtained inComparative Examples 1, 2, and 3 respectively. All the resultantabsorbent articles weighed 47 g.

[0356] The absorption amount of each of these absorbent articles in astate of lying face down was determined in the same way as of Example 4.The result of the absorption amount (g) in a state of lying face down isshown in Table 2 along with values of deterioration absorption indexunder load, substantial concentration absorption index, and static anddynamic deterioration concentration absorption indices. TABLE 2 Com-Com- Comp- parative parative arative Example Example Example 4 5 6Example Example Com- Com- Example Com- 4 5 parative parative 6 parativeWater- Water- Water- water- water- Water- water- absorbing absorbingabsorbing absorbing absorbing absorbing absorbing agent used agent 1agent 2 agent 1 agent 2 agent 3 agent 3 Deterioration 129.5 133.1 88.391.3 129.9 105.9 absorption index under load (g/g) Substantial 28.2 30.425.6 25.3 29.4 — concentration absorption index (g/g) Static 28.2 30.525.5 25.1 29.3 27.4 deterioration concentration absorption index (g/g)Dynamic 27.0 30.5 25.5 23.5 28.5 27.4 deterioration concentrationabsorption index (g/g) Absorption 275 288 250 250 288 268 amount instate of lying face down (g)

EXAMPLE 7

[0357] First, 75 weight parts of water-absorbing agent (1), as obtainedin Example 1, and 25 weight parts of wood-pulverized pulp were mixedtogether in a dry manner with a mixer. Next, the resultant mixture wasshaped into a web of the size of 120 mm×350 mm by pneumatically moldingthe mixture on a wire screen of 400 mesh (mesh size: 38 μm) with a batchtype pneumatic device. In addition, this web was pressed for 5 secondsunder a pressure of 2 kg/cm², thus obtaining an absorbent matter of aweight of about 500 g/m².

[0358] Next, a back sheet (liquid-impermeable sheet) of aliquid-impermeable polypropylene with a so-called leg gather, theabove-mentioned absorbent matter, and a top sheet (liquid-permeablesheet) of a liquid-permeable polypropylene were attached to each otherin this order with double coated tapes, and two so-called tape fastenerswere then provided to the resultant attached product, thus obtaining anabsorbent article (i.e. paper diaper). The weight of this absorbentarticle was 44 g.

[0359] This absorbent article was fitted up to each of four units ofso-called kewpie dolls (three units of which had a body length of 55 cmand a weight of 5 kg, and the other one unit had a body length of 65 cmand a weight of 6 kg), and these dolls were laid on their faces at roomtemperature of 37° C. Then, a tube was inserted between the absorbentarticle and the dolls, and 50 g of a physiological sodium chloridesolution containing L-ascorbic acid in a concentration of 0.005 wt % wasinjected through the tube every 90 minutes to a position correspondingto where urine is discharged from the human body. Then, this injectionoperation was ended when the injected physiological sodium chloridesolution began leaking without being absorbed by the absorbent article,and the amount of the physiological sodium chloride solution, as hadbeen injected until then, was measured, and the average value thereoffor the above-mentioned four units of kewpie dolls was regarded as theabsorption amount of the absorbent article in a state of lying facedown. The result of the absorption amount (g) in a state of lying facedown is shown in Table 3 along with values of deterioration absorptionindex under load, substantial concentration absorption index, and staticand dynamic deterioration concentration absorption indices.

EXAMPLES 8 and 9

[0360] Absorbent articles were obtained in the same way as of Example 7except that water-absorbing agent (1) was replaced with water-absorbingagents (2) and (3) as obtained in Examples 2 and 3 respectively. Boththe resultant absorbent articles weighed 44 g.

[0361] The absorption amount of each of these absorbent articles in astate of lying face down was determined in the same way as of Example 7.The result of the absorption amount (g) in a state of lying face down isshown in Table 3 along with values of deterioration absorption indexunder load, substantial concentration absorption index, and static anddynamic deterioration concentration absorption indices.

Comparative Examples 7, 8, and 9

[0362] Comparative absorbent articles were obtained in the same way asof Example 7 except that water-absorbing agent (1) was replaced withcomparative water-absorbing agents (1), (2), and (3) as obtained inComparative Examples 1, 2, and 3 respectively. All the resultantabsorbent articles weighed 44 g.

[0363] The absorption amount of each of these absorbent articles in astate of lying face down was determined in the same way as of Example 7.The result of the absorption amount (g) in a state of lying face down isshown in Table 3 along with values of deterioration absorption indexunder load, substantial concentration absorption index, and static anddynamic deterioration concentration absorption indices. TABLE 3 Com-Com- Com- parative parative parative Example Example Example 7 8 9Example Example Com- Com- Example Com- 7 8 parative parative 9 parativeWater- Water- Water- water- water- Water- water- absorbing absorbingabsorbing absorbing absorbing absorbing absorbing agent used agent 1agent 2 agent 1 agent 2 agent 3 agent 3 Deterioration 129.5 133.1 88.391.3 129.9 105.9 absorption index under load (g/g) Substantial 26.5 26.820.7 22.3 26.8 — concentration absorption index (g/g) Static 26.4 27.020.4 21.9 26.7 23.3 deterioration concentration absorption index (g/g)Dynamic 24.6 26.9 20.4 19.5 25.5 23.3 deterioration concentrationabsorption index (g/g) Absorption 275 275 238 250 288 268 amount instate of lying face down (g)

EXAMPLE 10

[0364] First, 60 weight parts of water-absorbing agent (1), as obtainedin Example 1, and 40 weight parts of wood-pulverized pulp were mixedtogether in a dry manner with a mixer. Next, the resultant mixture wasshaped into a web of the size of 120 mm×380 mm by pneumatically moldingthe mixture on a wire screen of 400 mesh (mesh size: 38 μm) with a batchtype pneumatic device. In addition, this web was pressed for 5 secondsunder a pressure of 2 kg/cm², thus obtaining an absorbent matter of aweight of about 530 g/m².

[0365] Next, a back sheet (liquid-impermeable sheet) of aliquid-impermeable polypropylene with a so-called leg gather, theabove-mentioned absorbent matter, and a top sheet (liquid-permeablesheet) of a liquid-permeable polypropylene were attached to each otherin this order with double coated tapes, and two so-called tape fastenerswere then provided to the resultant attached product, thus obtaining anabsorbent article (i.e. paper diaper). The weight of this absorbentarticle was about 47 g.

Comparative Example 10

[0366] A comparative absorbent article was obtained in the same way asof Example 10 except that water-absorbing agent (1) was replaced withcomparative water-absorbing agent (2) as obtained in Comparative Example2. The resultant absorbent article weighed about 47 g.

[0367] A test was carried out for 5 children of the age ranging from 1year and 8 months to 2 years and 4 months as follows. Ten absorbentarticles (as obtained in Example 10) and ten comparative absorbentarticles (as obtained in Comparative Example 10) were distributed toevery child. After each of the diapers had been used for one night, thediapers were collected to examine the amounts of urine as absorbed bythe diapers and whether urine leaked or not while the children wore thediapers. The data treatment was carried out by making calculations forabsorbent articles that absorbed 150 g or more of urine, thus excludingthe leakage that was, for example, caused by deviation of diapers fromtheir fit positions when being wore. The results are shown in Table 4.

[0368] The average amount of urine is, with regard to paper diapers thatabsorbed 150 g or more of urine, a value as given by dividing the totalamount of urine, as absorbed by such diapers, by the number of suchdiapers.

[0369] The average amount of urine in the case of leakage is, withregard to paper diapers that absorbed 150 g or more of urine, a value asgiven by dividing the total amount of urine, as absorbed by such diapersuntil the leakage occurred, by the number of such diapers that underwentthe leakage.

[0370] The leakage ratio is a ratio (percentage) of the number of paperdiapers that underwent the leakage, among paper diapers that absorbed150 g or more of urine, to the number of the paper diapers that absorbed150 g or more of urine. TABLE 4 Comparative Water-absorbingWater-absorbing water-absorbing agent used agent (1) agent (2) Averageamount 258 261 (g) of urine Average amount 355 324 (g) of urine in caseof leakage Leakage ratio (%)  8  12

[0371] With regard to the commercially available diapers (which werebought in the period of from April to September in 1998) as shown inTable 5, the following properties were calculated and are shown in Table5:

[0372] the weight ratio of the water-absorbent resin to the total of thewater-absorbent resin and the fibrous material, namely, thewater-absorbent resin concentration;

[0373] the properties of the water-absorbent resin, such as absorptioncapacity under no load, absorption capacity under load, absorptionspeed, water-soluble content, substantial absorption capacities (1)˜(2)under load, static deterioration absorption capacities (1)˜(4) underload, dynamic absorption capacity under load, dynamic deteriorationabsorption capacity under load, and deterioration absorption index underload; and

[0374] the properties of the absorbent matter, such as substantialconcentration absorption index and static and dynamic deteriorationconcentration absorption indices.

[0375] The way to measure each property is as follows:

[0376] (1) Water-absorbent Resin Concentration

[0377] Each of the above commercially available diapers was dried undervacuum at 60° C. for 16 hours. Then, the back sheet, the top sheet, thenonwoven fabric sheet, the paper, and the acquisition layer if any (someof the above diapers further include this acquisition layer consistingof the fibrous material) were all removed from each diaper to obtain anabsorbent layer mainly comprising the water-absorbent resin and thefibrous material. Then, the weight X (g) of the absorbent layer wasmeasured, and then the weight Y (g) of the water-absorbent resin, asincluded in the absorbent layer, was quantified, thus calculating thewater-absorbent resin concentration from the following equation:

water-absorbent resin concentration=Y/X.

[0378] (2) Properties of Water-absorbent Resin

[0379] The water-absorbent resin and the fibrous material, as includedin the absorbent matter of each commercially available diaper, wereseparated from each other and then dried under vacuum at 60° C. for 16hours. Then, measurement was made for the properties of thewater-absorbent resin, such as absorption capacity under no load,absorption capacity under load, absorption speed, water-soluble content,substantial absorption capacities (1)˜(2) under load, staticdeterioration absorption capacities (1)˜(4) under load, dynamicabsorption capacity under load, and dynamic deterioration absorptioncapacity under load, in the aforementioned way. In addition, thedeterioration absorption index under load of the water-absorbent resinis the total of the resultant measurement values of static deteriorationabsorption capacities (1)˜(4) under load and dynamic absorption capacityunder load.

[0380] (3) Properties of Absorbent Matter

[0381] The water-absorbent resin and the fibrous material, as includedin the absorbent matter of each commercially available diaper, wereseparated from each other and then dried under vacuum at 60° C. for 16hours. Then, calculation was made for the substantial concentrationabsorption index and the static and dynamic deterioration concentrationabsorption indices. TABLE 5 Trade name Moony Pampers Dri- PowerSara-Sara Pampers HUGGIES Bottoms Slim Care Premium for Boys SupremeSize L L Maxi Plus Maxi 4 Maker UNI- Procter & Kimbery- Paragon CHARMGamble Far Procter & Clark Trade K.K. East, Inc. Gamble CorporationBrands Purchase country Japan Japan Germany UK USA Water-absorbent resinconcentration 0.4 0.49 0.5 0.4 0.4 Absorption capacity under no load(g/g) 37 29 31 28 31 Absorption capacity under load (g/g) 8 23 24 15 24Absorption speed (sec) 29 71 45 38 36 Water-soluble content (%) 7.2 9.712 9.0 9.1 Substantial absorption capacity (1) 15.7 20.0 20.6 15.8 21.4under load (g/g) Substantial absorption capacity (2) 15.2 19.4 19.4 15.419.7 under load (g/g) Static deterioration absorption capacity 15.2 16.317.2 15.3 17.4 (1) under load (g/g) Static deterioration absorptioncapacity 15.5 18.0 21.5 15.6 21.2 (2) under load (g/g) Staticdeterioration absorption capacity 15.3 16.4 17.5 15.4 17.2 (3) underload (g/g) Static deterioration absorption capacity 15.4 15.6 16.4 25.724.3 (4) under load (g/g) Dynamic absorption capacity under load 15.220.0 22.0 15.8 21.2 (g/g) Dynamic deterioration absorption 15.4 15.416.1 15.3 17.5 capacity under load (g/g) Deterioration absorption indexunder 76.8 81.7 88.7 87.3 97.6 load (g/g) Absorbent Substantialconcentration 28.3 24.3 25.2 23.0 26.5 matter absorption index (g/g)Static deterioration 28.3 22.8 24.1 22.9 25.6 concentration absorptionindex (g/g) Dynamic deterioration 28.4 22.3 23.6 22.9 25.6 concentrationabsorption index (g/g)

[0382] Hereinafter, examples of some preferred embodiments of thepresent invention water-absorbing agent with excellent urine resistanceand those of the production process therefor are described in detail.However, the present invention is not limited to these examples. Inaddition, in the examples and comparative examples, unless otherwisenoted, the units “%” and “part(s)” denote those by weight.

[0383] Incidentally, the properties of the water-absorbing agent, suchas water absorption amount, water-soluble content, and soluble contentas eluted into artificial urine, were measured by the methods below.

[0384] (1) Water Absorption Amount of Water-absorbing Agent

[0385] First, 0.2 g of water-absorbent resin was uniformly placed into atea bag type bag (6 cm×6 cm), of which the opening was then sealed byheating, and the bag was then immersed into a physiological sodiumchloride solution. Sixty minutes later, the bag was drawn up and thendrained at 250 G for 3 minutes with a centrifuge, and the weight W₁ (g)of the bag was then measured. On the other hand, the same procedure wascarried out using no water-absorbent resin, and the resultant weight W₀(g) was measured. Thus, the water absorption amount (g/g) was calculatedfrom these weights W₁ and W₀ in accordance with the following equation:

water absorption amount (g/g)=(W ₁ −W ₀)/(weight (g) of water-absorbentresin).

[0386] (2) Soluble Content as Eluted from Water-absorbing Agent

[0387] First of all, 1 g of water-absorbing agent was swollen with 25 mlof artificial urine in a 100 ml beaker, and the beaker was allowed tostand stationary at 37° C. for 16 hours. Next, the resultant swollen gelwas dispersed into 975 ml of deionized water and stirred for 1 hour, andthen filtered with a filter paper. The resultant filtrate was titratedby colloid titration to determine the soluble content (%) as eluted fromthe water-absorbing agent.

[0388] The composition of the artificial urine is as follows: urea 1.9%sodium chloride 0.8% magnesium chloride 0.1% calcium chloride 0.1%

[0389] (3) Soluble Content as Deteriorated and Eluted fromWater-absorbing Agent

[0390] First of all, 1 g of water-absorbing agent was swollen with 25 mlof artificial urine, containing L-ascorbic acid in a concentration of0.005%, in a 100 ml beaker, and then allowed to stand stationary at 37°C. for 16 hours. Next, the resultant swollen gel was dispersed into 975ml of deionized water to rinse the eluted soluble contents withdeionized water. The dispersion was stirred for 1 hour and then filteredwith a filter paper. The resultant filtrate was titrated by colloidtitration to determine the soluble content (%) as deteriorated andeluted from the water-absorbing agent.

[0391] (4) Absorption Capacity Under Load

[0392] The absorption capacity under a load was determined using ameasurement apparatus of FIG. 1. As is shown in FIG. 1, the measurementapparatus comprises: a scale 1; a vessel 2 of a predetermined capacityas mounted on the scale 1; an air-inhaling pipe sheet 3; an introducingtube 4; a glass filter 6; and a measurement part 5 as mounted on theglass filter 6. The vessel 2 has an opening part 2 a on the top and anopening part 2 b on the side. The air-inhaling pipe 3 is inserted in theopening part 2 a, and the introducing tube 4 is fitted to the openingpart 2 b. In addition, the vessel 2 contains a predetermined amount of0.9 wt % aqueous sodium chloride solution 12 (hereinafter referred to asphysiological sodium chloride solution). The lower part of theair-inhaling pipe 3 is submerged in the physiological sodium chloridesolution 12. The air-inhaling pipe 3 is set to keep the internalpressure of the vessel 2 nearly atmospheric. The glass filter 6 isformed in a diameter of 55 mm. The vessel 2 and the glass filter 6 areconnected to each other through the introducing tube 4 made of siliconeresin. In addition, the position and level of the glass filter 6 arefixed relative to the vessel 2. The measurement part 5 comprises: afilter paper 7; a supporting cylinder 9; a wire net 10 as attached tothe bottom of the supporting cylinder 9; and a weight 11. Themeasurement part 5 is formed by mounting the filter paper 7 and thesupporting cylinder 9 (i.e. wire net 10) in this order on the glassfilter 6. The wire net 10 is made of stainless steel and has a mesh sizeof 400 mesh. The level of the upper face of the wire net 10, namely, ofthe contact face of the wire net 10 with a water-absorbing agent 15, isset so as to be as high as the level of the lower end face 3 a of theair-inhaling pipe 3. On the wire net 10, a predetermined amount ofwater-absorbing agent is uniformly spread. The weight 11 is adjusted inweight such that a load of 0.7 psi can uniformly be applied to the wirenet 10, namely, to the water-absorbing agent 15.

[0393] The absorption capacity under a load was measured with themeasurement apparatus having the above-mentioned constitution. Themeasurement method is hereinafter explained.

[0394] First, predetermined preparatory operations were made, in which,for example, a predetermined amount of physiological sodium chloridesolution 12 was placed into the vessel 2, and the air-inhaling pipe 3was inserted into the vessel 2. Next, the filter paper 7 was mounted onthe glass filter 6. On the other hand, in parallel with these mountingoperations, 0.9 g of water-absorbing agent was uniformly spread insidethe supporting cylinder 9, namely, on the wire net 10, and the weight 11was put on the water-absorbing agent 15. Next, the wire net 10, namely,the supporting cylinder 9 (in which the water-absorbing agent 15 and theweight 11 were put), was mounted on the filter paper 7 such that thecenter line of the supporting cylinder 9 would conform with that of theglass filter 6. Then, the weight of the physiological sodium chloridesolution, as absorbed by the water-absorbing agent 15 over a period of60 minutes since the supporting cylinder 9 had been mounted on thefilter paper 7, was determined from a value as measured with the scale1. In addition, the same procedure as the above was carried out using nowater-absorbing agent 15, and the weight of the physiological sodiumchloride solution, as absorbed by materials other than thewater-absorbing agent, such as the filter paper 7, was determined from avalue as measured with the scale 1 and regarded as the blank value. Theabsorption amount under a load was calculated from the followingequation:

absorption capacity (g/g) under load=(water absorption amount after 60minutes−blank)/(weight of water-absorbing agent).

[0395] (5) Average Particle Diameter of Water-absorbing Agent

[0396] The water-absorbing agent was sieved and classified with screensof 850 μm, 600 μm, 500 μm, 425 μm, 300 μm, 220 μm, 150 μm, and 105 μm,and then the percentage of the residue, R, was plotted on logarithmicprobability paper to regard a particle diameter corresponding to R=50%as the average particle diameter.

[0397] (6) Water Content (on the Wet Basis) of Water-absorbent Resin

[0398] About 1 g of water-absorbent resin was heated in an oven of 105°C. for 3 hours, and the weight W (g) of the water-absorbent resin weremeasured before and after heating, and the water content (wt %) (on thewet basis) was calculated in accordance with the following equation:

water content (wt %)=(W _(before) (g)−W _(after) (g))/W _(before) (g)

[0399] wherein:

[0400] W_(before) is the weight of the water-absorbent resin beforedrying; and W_(after) is the weight of the water-absorbent resin afterdrying.

Referential Example 1

[0401] An aqueous monomer solution was prepared by mixing 67.0 parts ofa 37% aqueous sodium acrylate solution, 10.2 parts of acrylic acid,0.079 parts of polyethylene glycol diacrylate (average number ofpolyethylene oxide units: 8), and 22.0 parts of water together. Nitrogenwas blown into the above aqueous monomer solution in a vat, thusreducing the concentration of dissolved oxygen in the solution to 0.1ppm or below.

[0402] Then, the temperature of the solution was adjusted to 18° C.under nitrogen atmosphere. Next, 0.16 parts of a 5% aqueous sodiumpersulfate solution, 0.16 parts of a 5% aqueous 2,2′-azobis(2-amidinopropane) hydrochloride solution, 0.15 parts of a 0.5% aqueousL-ascorbic acid solution, and 0.17 parts of a 0.35% aqueous hydrogenperoxide solution were dropped in sequence under stirring.

[0403] Immediately after the dropping of hydrogen peroxide, apolymerization reaction got started, and after another 10 minutes, thetemperature of the monomer reached the peak temperature. The peaktemperature was 85° C. Then, the vat was immersed into a hot water bathof 80° C. and aged for 10 minutes.

[0404] The resultant transparent hydrogel was crushed with a meatchopper and then dried at 180° C. for 30 minutes.

[0405] The resultant dry product was pulverized with a pulverizingmachine and then classified into what passed through a screen of 500 μm,but remained on a screen of 105 μm, thus obtaining water-absorbent resin(A).

EXAMPLE 2-1

[0406] A composition solution, comprising 0.001 part of pentasodiumdiethylenetriaminepentaacetate, 0.05 parts of ethylene glycol diglycidylether, 1 part of propylene glycol, 3 parts of water, and 1 part ofisopropyl alcohol, was mixed with 100 parts of water-absorbent resin (A)as obtained in Referential Example above, and the resultant mixture washeated at 180° C. for 40 minutes, thus obtaining a water-absorbingagent. The performance evaluation results of the resultantwater-absorbing agent (E2-1) are shown in Table 2-1.

EXAMPLE 2-2

[0407] A water-absorbing agent according to the present invention wasobtained in the same way as of Example 2-1 except that the amount ofpentasodium diethylenetriaminepentaacetate was changed to 0.01 part. Theperformance evaluation results of the resultant water-absorbing agent(E2-2) are shown in Table 2-1.

EXAMPLE 2-3

[0408] A water-absorbing agent according to the present invention wasobtained in the same way as of Example 2-1 except that the amount ofpentasodium diethylenetriaminepentaacetate was changed to 0.1 part. Theperformance evaluation results of the resultant water-absorbing agent(E2-3) are shown in Table 2-1.

EXAMPLE 2-4

[0409] A water-absorbing agent according to the present invention wasobtained in the same way as of Example 2-1 except that 0.01 part ofhexasodium triethylenetetraamine hexaacetate was used instead ofpentasodium diethylenetriaminepentaacetate. The performance evaluationresults of the resultant water-absorbing agent (E2-4) are shown in Table2-1.

EXAMPLE 2-5

[0410] A water-absorbing agent according to the present invention wasobtained in the same way as of Example 2-1 except that 0.01 part ofcyclohexanediaminetetraacetate was used instead of pentasodiumdiethylenetriaminepentaacetate. The performance evaluation results ofthe resultant water-absorbing agent (E2-5) are shown in Table 2-1.

Comparative Example 2-1

[0411] A comparative water-absorbing agent was obtained in the same wayas of Example 2-1 except that pentasodium diethylenetriaminepentaacetatewas not added. The performance evaluation results of the resultantcomparative water-absorbing agent (R2-1) are shown in Table 2-1.

Referential Example 2

[0412] An aqueous monomer solution was prepared by mixing 81.8 parts ofa 38% aqueous sodium acrylate solution, 7.7 parts of acrylic acid, 0.038parts of trimethylolpropane triacrylate and 9.8 parts of water together.

[0413] Nitrogen was blown into the above aqueous monomer solution in atwin-arm kneader as equipped with a jacket, thus removing dissolvedoxygen from the solution. Then, the temperature of the aqueous monomersolution was adjusted to 22° C.

[0414] Next, 0.60 parts of a 10% aqueous sodium persulfate solution and0.30 parts of a 0.1% aqueous L-ascorbic acid solution were added understirring. One minute later than this addition, the aqueous monomersolution began clouding and its temperature began rising. After another20 minutes, the temperature reached the peak temperature, and thesolution was then aged for 20 minutes under stirring. The peaktemperature was 96° C.

[0415] After the aging had finished, the resultant gel was got out anddried at 170° C. for 65 minutes. The resultant dry polymer waspulverized and then sieved with a screen of 850 μm, thus obtainingwater-absorbent resin (B).

EXAMPLE 2-6

[0416] A composition solution, comprising 0.001 parts ofcyclohexanediaminetetraacetate, 0.5 parts of ethylene carbonate, 3 partsof water, and 3 parts of isopropyl alcohol, was mixed with 100 parts ofwater-absorbent resin (B), and the resultant mixture was heated at 190°C. for 50 minutes, thus obtaining a water-absorbing agent. Theperformance evaluation results of the resultant water-absorbing agent(E2-6) are shown in Table 2-1.

EXAMPLE 7

[0417] A water-absorbing agent according to the present invention wasobtained in the same way as of Example 2-6 except that 0.5 parts of1,4-butanediol was used instead of ethylene carbonate. The performanceevaluation results of the resultant water-absorbing agent (E2-7) areshown in Table 2-1.

Comparative Example 2-2

[0418] A comparative water-absorbing agent was obtained in the same wayas of Example 2-6 except that cyclohexanediaminetetraacetate was notadded. The performance evaluation results of the resultant comparativewater-absorbing agent (R2-2) are shown in Table 2-1.

Comparative Example 2-3

[0419] A comparative water-absorbing agent was obtained in the same wayas of Example 2-7 except that cyclohexanediaminetetraacetate was notadded. The performance evaluation results of the resultant comparativewater-absorbing agent (R2-3) are shown in Table 2-1. TABLE 2-1 SolubleWater content as Water- Water absorp- Soluble deteri- absorb- absorp-tion content orated Ex- ing tion amount as and ample agent amount underload eluted eluted No. No. (g/g) (g/g) (%) (%) Ex- 2-1 E2-1 34.4 28.811.1 13.8 ample 2-2 E2-2 34.3 28.1 11.1 12.0 2-3 E2-3 34.4 27.9 11.211.3 2-4 E2-4 34.1 28.2 10.9 11.5 2-5 E2-5 34.2 27.8 11.0 11.8 2-6 E2-628.0 22.0 10.5 10.9 2-7 E2-7 27.8 23.5 10.8 11.5 Com- 2-1 R2-1 34.0 28.011.1 24.5 para- 2-2 R2-2 28.1 22.1 10.5 25.6 tive 2-3 R2-3 28.0 23.410.7 26.0 Example

EXAMPLE 3-1

[0420] One hundred weight parts of comparative water-absorbing agent(R2-1), as obtained in comparative example 2-1, was sprayed with a mixedsolution, comprising 0.001 part of pentasodiumdiethylenetriaminepentaacetate and 3 parts of water, and therebygranulated, and then dried at 80° C., thus obtaining a water-absorbingagent. The evaluation results of the resultant water-absorbing agent(E3-1) are shown in Table 3-1.

EXAMPLE 3-2

[0421] A water-absorbing agent was obtained in the same way as ofExample 3-1 except that the amount of pentasodiumdiethylenetriaminepentaacetate was changed to 0.1 part. The evaluationresults of the resultant water-absorbing agent (E3-2) are shown in Table3-1.

EXAMPLE 3-3

[0422] A water-absorbing agent was obtained in the same way as ofExample 3-1 except that 0.001 part of hexasodium triethylenetetraamine hexaacetate was used instead of pentasodiumdiethylenetriaminepentaacetate. The evaluation results of the resultantwater-absorbing agent (E3-3) are shown in Table 3-1.

Comparative Example 3-1

[0423] Water-absorbing agent (A) was regarded as comparativewater-absorbing agent (R3-1) as it was. The performance evaluationresults of comparative water-absorbing agent (R3-1) are shown in Table3-1.

Comparative Example 3-2

[0424] A comparative water-absorbing agent was obtained in the same wayas of Example 3-1 except that 100 weight parts of comparativewater-absorbing agent (R2-1) was mixed with only 3 weight parts ofwater. The performance evaluation results of comparative water-absorbingagent (R3-2) are shown in Table 3-1. TABLE 3-1 Soluble Soluble Water-Water content content as Absorption Average absorbing absorption asdeteriorated capacity particle Example agent amount eluted and elutedunder load diameter No. No. (g/g) (%) (%) (g/g) (μm) Ex- 3-1 E3-1 34 1115 27 420 ample 3-2 E3-2 33 11 11 27 420 3-3 E3-3 33 11 12 27 420 Com-3-1 R3-1 34 11 25 27 280 parative 3-2 R3-2 34 11 29 26 420 Example

EXAMPLE 4-1

[0425] A composition solution, comprising 0.01 part of tetrasodiumN,N′-bis(1,2-dicarboxyethyl)-ethylenediamine, 0.05 parts of ethyleneglycol diglycidyl ether, 1 part of propylene glycol, 3 parts of water,and 1 part of isopropyl alcohol, was mixed with 100 parts ofwater-absorbent resin (A) as obtained in Referential Example 1, and theresultant mixture was heated at 180° C. for 40 minutes, thus obtaining awater-absorbing agent. The performance evaluation results of theresultant water-absorbing agent (E4-1) are shown in Table 4-1.

Comparative Example 4-1

[0426] A comparative water-absorbing agent was obtained in the same wayas of Example 4-1 except that either 0.01 part of tetrasodiumN,N′-bis(1,2-dicarboxyethyl)-ethylenediamine or 0.05 parts of ethyleneglycol diglycidyl 15 ether was not added. The performance evaluationresults of the resultant comparative water-absorbing agent (R4-1) areshown in Table 4-1. The water content of comparative water-absorbingagent (R4-1) was 1 weight % or below.

EXAMPLE 4-2 EXAMPLE 4-6

[0427] A water-absorbing agent was obtained in the same way as ofExample 4-2 except that 0.001 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine was replaced with 0.1part of sodium polymaleate with a molecular weight of about 10,000. Theperformance evaluation results of the resultant water-absorbing agent(E4-6) are shown in Table 4-1.

EXAMPLE 4-7

[0428] A water-absorbing agent was obtained in the same way as ofExample 4-2 except that 0.001 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine was replaced with0.01 part of tetrasodium N,N-dicarboxymethyl-L-glutamate. Theperformance evaluation results of the resultant water-absorbing agent(E4-7) are shown in Table 4-1.

EXAMPLE 4-8

[0429] A water-absorbing agent was obtained in the same way as ofExample 4-2 except that 0.001 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine was replaced withtetrasodium (R,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine. Theperformance evaluation results of the resultant water-absorbing agent(E4-8) are shown in Table 4-1.

Comparative Example 4-2

[0430] A comparative water-absorbing agent was obtained in the same wayas of Example 4-2 except that 0.001 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine was replaced with0.01 part of acetylacetone. The performance evaluation results of theresultant comparative water-absorbing agent (R4-2) are shown in Table4-1.

EXAMPLE 4-9

[0431] A composition solution, comprising 0.01 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine, 0.5 parts ofethylene carbonate, 3 parts of water, and 3 parts of isopropyl alcohol,was mixed with 100 parts of water-absorbent resin (B) as obtained inReferential Example 2, and the resultant mixture was heated at 190° C.for 50 minutes, thus obtaining a water-absorbing agent. The performanceevaluation results of the resultant water-absorbing agent (E4-9) areshown in Table 4-1.

EXAMPLE 4-10

[0432] A water-absorbing agent was obtained in the same way as ofExample 4-9 except that ethylene carbonate was replaced with 0.5 partsof 1,4-butanediol. The performance evaluation results of the resultantwater-absorbing agent (E4-10) are shown in Table 4-1.

Comparative Example 4-3

[0433] A comparative water-absorbing agent was obtained in the same wayas of Example 4-9 except that 0.01 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine was not added. Theperformance evaluation results of the resultant comparativewater-absorbing agent (R4-3) are shown in Table 4-1.

Comparative Example 4-4

[0434] A comparative water-absorbing agent (R4-4) was obtained in thesame way as of Example 4-10 except that 0.01 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine was not added. Theperformance evaluation results of the resultant comparativewater-absorbing agent (R4-4) are shown in Table 4-1.

EXAMPLE 4-11

[0435] First of all, 2 g of absorbing agent (E4-2), as obtained inExample 4-2, was uniformly spread and sandwiched between two laminatepulp sheets (weight 150 g/m², density 0.1 g/cm³, size 200 mm×140 mm),thus obtaining an absorbent matter. This absorbent matter was interposedbetween a sheet of polyethylene film and a sheet of polypropylenenonwoven fabric, thus obtaining a body-fluid-absorbent article.

[0436] Next, 100 g of artificial urine containing L-ascorbic acid in aconcentration of 0.005% was poured onto the nonwoven fabric side of theresultant body-fluid-absorbent article and allowed to be absorbed. Thisbody-fluid-absorbent article was left stationary at 37° C. for 8 hours,and then ten paper towels of 23 cm×23 cm were laminated on the nonwovenfabric side of the body-fluid-absorbent article. A pressure of 40 g/cm²is applied for 1 minute, and the amount of the artificial urine asabsorbed by the paper towels was measured as the desorption amount. Inaddition, the state of the resultant swollen gel was observed with thenaked eye to evaluate the deteriorated state of the gel in three classesof ◯, Δ, X. The results are shown in Table 4-2.

Comparative Example 4-5

[0437] A comparative body-fluid-absorbent article was obtained in thesame way as of Example 4-11 except that comparative water-absorbingagent (R4-1) was used instead of water-absorbing agent (E4-2). Theevaluation results of the resultant comparative body-fluid-absorbentarticle are shown in Table 4-2.

Comparative Example 4-6

[0438] A comparative body-fluid-absorbent article was obtained in thesame way as of Example 4-11 except that comparative water-absorbingagent (R4-2) was used instead of water-absorbing agent (E4-2). Theevaluation results of the resultant comparative body-fluid-absorbentarticle are shown in Table 4-2. TABLE 4-1 Water Soluble Water- Waterabsorp- content as absorb- absorp- tion Soluble deteri- Ex- ing tionamount content orated ample agent amount under load as eluted and elutedNo. No. (g/g) (g/g) (%) (%) Ex- 4-1 E4-1 34 29 11 13 ample 4-2 E4-2 3428 11 13 4-3 E4-3 34 28 11 11 4-4 E4-4 34 28 11 18 4-5 E4-5 34 28 11 144-6 E4-6 34 28 11 20 4-7 E4-7 34 28 11 21 4-8 E4-8 34 28 11 13 4-9 E4-928 22 11 11 4-10 E4-10 28 24 11 12 Com- 4-1 R4-1 34 28 11 25 para- 4-2R4-2 34 28 11 26 tive 4-3 R4-3 28 22 11 26 Ex- 4-4 R4-4 28 23 11 26ample

[0439] TABLE 4-2 Water- absorbing agent Desorption amount State ofswollen gel No. (g) (Note 1) Example 4-11 E4-2 1 ◯ Comparative R4-1 6 ΔExample 4-5 Comparative R4-2 8 X Example 4-6

[0440] Various details of the invention may be changed without departingA composition solution, comprising 0.001 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine and 3 parts of water,was mixed with 100 parts of comparative water-absorbing agent (R4-1) asobtained in Comparative Example 4-1, and the resultant mixture was driedat 80° C. for 20 minutes, thus obtaining a water-absorbing agent. Theperformance evaluation results of the resultant water-absorbing agent(E4-2) are shown in Table 4-1.

EXAMPLE 4-3

[0441] A water-absorbing agent was obtained in the same way as ofExample 4-2 except that the amount of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine was changed to 0.01part. The performance evaluation results of the resultantwater-absorbing agent (E4-3) are shown in Table 4-1.

EXAMPLE 4-4

[0442] A water-absorbing agent was obtained in the same way as ofExample 4-2 except that a composition solution, comprising 0.1 part oftetrasodium N-(1,2-dicarboxy-2-hydroxyethyl)-aspartate and 5 parts ofwater, was mixed with 100 parts of comparative water-absorbing agent(R4-1). The performance evaluation results of the resultantwater-absorbing agent (E4-4) are shown in Table 4-1.

EXAMPLE 4-5

[0443] A water-absorbing agent was obtained in the same way as ofExample 4-2 except that 0.001 part of trisodium(S,S)-N,N′-bis(1,2-dicarboxyethyl)-ethylenediamine was replaced with0.01 part of tetrasodiumN,N′-bis(1,2-dicarboxy-2-hydroxyethyl)-ethylenediamine. The performanceevaluation results of the resultant water-absorbing agent (E4-5) areshown in Table 4-1. from its spirit not its scope. Furthermore, theforegoing description of the preferred embodiments according to thepresent invention is provided for the purpose of illustration only, andnot for the purpose of limiting the invention as defined by the appendedclaims and their equivalents.

What is claimed is:
 1. A water-absorbing agent, having an absorptioncapacity of 30 (g/g) or more under no load and static deteriorationabsorption capacity (1) of 20 (g/g) or more under a load, wherein staticdeterioration absorption capacity (1) under a load is an absorptioncapacity of the water-absorbing agent as determined by the followingsequential steps of: swelling a water-absorbing agent to 15 (g/g) with aphysiological sodium chloride solution containing L-ascorbic acid in aconcentration of 0.005 weight %; leaving the water-absorbing agent insuch a swollen state for 6 hours; allowing the swollen water-absorbingagent to absorb the physiological sodium chloride solution for another 1hour in a state where a load of 50 g/cm² is mounted on the swollenwater-absorbing agent; and measuring the weight of the resultant swollengel.
 2. A water-absorbing agent according to claim 1, wherein staticdeterioration absorption capacity (1) under a load is 23 (g/g) or more.3. A water-absorbing agent according to claim 1, wherein the absorptioncapacity under no load is 33 (g/g) or more.
 4. A water-absorbing agentaccording to claim 1, having an absorption speed of 20˜80 (sec) and awater-soluble content of 1˜15 weight %.
 5. A water-absorbing agent,having an absorption capacity of 30 (g/g) or more under no load and adynamic deterioration absorption capacity of 20 (g/g) or more under aload, wherein the dynamic deterioration absorption capacity under a loadis an absorption capacity of the water-absorbing agent as determined bythe following sequential steps of: swelling a water-absorbing agent to15 (g/g) with a physiological sodium chloride solution containingL-ascorbic acid in a concentration of 0.005 weight %; leaving thewater-absorbing agent in such a swollen state for 4 hours; dynamicallydamaging the swollen water-absorbing agent; allowing the dynamicallydamaged water-absorbing agent to absorb the physiological sodiumchloride solution for another 1 hour in a state where a load of 50 g/cm²is mounted on the swollen water-absorbing agent; and measuring theweight of the resultant swollen gel.
 6. A water-absorbing agentaccording to claim 5, wherein dynamic deterioration absorption capacityunder a load is 23 (g/g) or more.
 7. A water-absorbing agent accordingto claim 5, wherein the absorption capacity under no load is 33 (g/g) ormore.
 8. A water-absorbing agent according to claim 5, having anabsorption speed of 20˜80 (sec) and a water-soluble content of 1˜15weight %.
 9. A water-absorbing agent, having an absorption capacity of30 (g/g) or more under no load and static deterioration absorptioncapacity (4) of 30 (g/g) or more under a load, wherein staticdeterioration absorption capacity (4) under a load is an absorptioncapacity of the water-absorbing agent as determined by the followingsequential steps of: swelling a water-absorbing agent to 15 (g/g) with aphysiological sodium chloride solution containing L-ascorbic acid in aconcentration of 0.05 weight %; leaving the water-absorbing agent insuch a swollen state for 6 hours; allowing the swollen water-absorbingagent to absorb the physiological sodium chloride solution for another 1hour in a state where a load of 20 g/cm² is mounted on the swollenwater-absorbing agent; and measuring the weight of the resultant swollengel.
 10. A water-absorbing agent according to claim 9, wherein staticdeterioration absorption capacity (4) under a load is 32 (g/g) or more.11. A water-absorbing agent according to claim 9, wherein the absorptioncapacity under no load is 33 (g/g) or more.
 12. A water-absorbing agentaccording to claim 9, having an absorption speed of 20˜80 (sec) and awater-soluble content of 1˜15 weight %.
 13. An absorbent matter,comprising the water-absorbing agent as recited in claim 1 and a fibrousbase material, wherein the weight ratio of the water-absorbing agent tothe total of the water-absorbing agent and the fibrous base material is0.4 or more.
 14. An absorbent matter according to claim 13, wherein thewater-absorbing agent has a static deterioration concentrationabsorption index of 23 or more of equation (1) below: staticdeterioration concentration absorption index=X(1−α)+Yα  (1) wherein: Xis the absorption capacity (g/g) under no load of the water-absorbingagent; Y is static deterioration absorption capacity (1) (g/g) under aload as recited in claim 1 of the water-absorbing agent; and α is theweight ratio of the water-absorbing agent to the total of thewater-absorbing agent and the fibrous base material.
 15. An absorbentmatter according to claim 13, wherein the water-absorbing agent has anabsorption speed of 20˜80 (sec) and a water-soluble content of 1˜15weight %.
 16. An absorbent matter, comprising the water-absorbing agentas recited in claim 5 and a fibrous base material, wherein the weightratio of the water-absorbing agent to the total of the water-absorbingagent and the fibrous base material is 0.4 or more.
 17. An absorbentmatter according to claim 16, wherein the water-absorbing agent has adynamic deterioration concentration absorption index of 23 or more ofequation (2) below: dynamic deterioration concentration absorptionindex=X(1−γ)+Aγ  (2) wherein: X is the absorption capacity (g/g) underno load of the water-absorbing agent; A is the dynamic deteriorationabsorption capacity (g/g) under a load as recited in claim 5 of thewater-absorbing agent; and γ is the weight ratio of the water-absorbingagent to the total of the water-absorbing agent and the fibrous basematerial.
 18. An absorbent matter according to claim 16, wherein thewater-absorbing agent has an absorption speed of 20˜80 (sec) and awater-soluble content of 1˜15 weight %.
 19. An absorbent matter,comprising the water-absorbing agent as recited in claim 9 and a fibrousbase material, wherein the weight ratio of the water-absorbing agent tothe total of the water-absorbing agent and the fibrous base material is0.4 or more.
 20. An absorbent matter according to claim 19, wherein thewater-absorbing agent has an absorption speed of 20˜80 (sec) and awater-soluble content of 1˜15 weight %.
 21. An absorbent article,comprising: an absorbent layer including the absorbent matter as recitedin claim 13; a liquid-permeable surface sheet; and a liquid-impermeableback sheet.
 22. An absorbent article according to claim 21, wherein thewater-absorbing agent has a static deterioration concentrationabsorption index of 23 or more of equation (1) below: staticdeterioration concentration absorption index=X(1−α)+Yα  (1) wherein: Xis the absorption capacity (g/g) under no load of the water-absorbingagent; Y is static deterioration absorption capacity (1) (g/g) under aload as recited in claim 1 of the water-absorbing agent; and α is theweight ratio of the water-absorbing agent to the total of thewater-absorbing agent and the fibrous base material.
 23. An absorbentarticle according to claim 21, wherein the water-absorbing agent has anabsorption speed of 20˜80 (sec) and a water-soluble content of 1˜15weight %.
 24. An absorbent article, comprising: an absorbent layerincluding the absorbent matter as recited in claim 16; aliquid-permeable surface sheet; and a liquid-impermeable back sheet. 25.An absorbent article according to claim 24, wherein the water-absorbingagent has a dynamic deterioration concentration absorption index of 23or more of equation (2) below: dynamic deterioration concentrationabsorption index=X(1−γ)+Aγ  (2) wherein: X is the absorption capacity(g/g) under no load of the water-absorbing agent; A is the dynamicdeterioration absorption capacity g/g) under a load as recited in claim5 of the water-absorbing agent; and γ is the weight ratio of thewater-absorbing agent to the total of the water-absorbing agent and thefibrous base material.
 26. An absorbent article according to claim 24,wherein the water-absorbing agent has an absorption speed of 20˜80 (sec)and a water-soluble content of 1˜15 weight %.
 27. An absorbent article,comprising: an absorbent layer including the absorbent matter as recitedin claim 19; a liquid-permeable surface sheet; and a liquid-impermeableback sheet.
 28. An absorbent article according to claim 27, wherein thewater-absorbing agent has an absorption speed of 20˜80 (sec) and awater-soluble content of 1˜15 weight %.
 29. An absorption propertymeasurement process, characterized in that a liquid containing areducible substance is used as a liquid to be absorbed in a process formeasuring at least one absorption property selected from the groupconsisting of: absorption properties of a water-absorbing agent under aload; absorption properties of an absorbent matter of which the weightratio of a water-absorbing agent to the total of the water-absorbingagent and a fibrous base material is 0.4 or more; and absorptionproperties of an absorbent article including the above absorbent matter.30. An absorption property measurement process according to claim 29,wherein the reducible substance is an ascorbic acid or its salts.
 31. Aproduction process for a water-absorbing agent, comprising the step ofmixing an ion blocking agent and a surface-crosslinking agent, which isreactable upon a carboxyl group, with a water-absorbent resin having acarboxyl group.
 32. A production process for a water-absorbing agentaccording to claim 31, wherein 0.0001˜10 weight parts of the ionblocking agent and 0.01˜10 weight parts of the surface-crosslinkingagent are mixed with 100 weight parts of the water-absorbent resin. 33.A production process for a water-absorbing agent according to claim 31,wherein 0.01˜10 weight parts of water is further mixed with 100 weightparts of the water-absorbent resin in the mixing step.
 34. A productionprocess for a water-absorbing agent according to claim 31, wherein theion blocking agent is at least one compound selected from the groupconsisting of aminocarboxylic acids with at least three carboxyl groupsand their salts.
 35. A production process for a water-absorbing agentaccording to claim 31, which further comprises the step of heating themixture, resultant from the mixing step, at 100˜230° C.
 36. A productionprocess for a water-absorbing agent, comprising the steps of:crosslinking the neighborhood of the surface of a water-absorbent resinwhich is obtained by polymerizing a monomer component including anunsaturated carboxylic acid in the presence of an internal-crosslinkingagent; and adding water and an ion blocking agent to the resultantsurface-crosslinked water-absorbent resin, thus granulating thewater-absorbent resin.
 37. A production process for a water-absorbingagent according to claim 36, wherein 0.1˜20 weight parts of water and0.0001˜10 weight parts of the ion blocking agent are added to 100 weightparts of the surface-crosslinked water-absorbent resin.
 38. A productionprocess for a water-absorbing agent according to claim 36, wherein theion blocking agent is at least one compound selected from the groupconsisting of aminocarboxylic acids with at least three carboxyl groupsand their salts.
 39. A production process for a water-absorbing agentaccording to claim 36, wherein the surface-crosslinked water-absorbentresin has a water content of 20 weight % or less (on the wet basis). 40.A production process for a water-absorbing agent according to claim 36,wherein the surface-crosslinked water-absorbent resin has an absorptioncapacity of at least 20 (g/g) under a load.
 41. A water-absorbing agent,obtained by a process including the step of adding to a water-absorbentresin at least one chelating agent selected from the group consisting ofcompounds of general formulae (1) and (2) and maleic hydrophilicpolymers (including salts) (3), wherein general formula (1) is:

wherein: n, X¹, and R¹˜R³ denote the following numbers and structures:

and wherein general formula (2) is:

wherein: m, X², and R⁵˜R⁸ denote the following numbers and structures:


42. A water-absorbing agent according to claim 41, wherein 0.00001˜30weight parts of the chelating agent is added to 100 weight parts of thewater-absorbent resin.
 43. A body-fluid-absorbent article, comprisingthe water-absorbing agent as recited in claim 41.